Dear community members,

I've managed to add a new state and customize workflow. Though it was not difficult to add a new state, extending existing views and workflows with minimum redundancy (i.e., not re-writing the same codes to preserve original behavior linked to other states) was a bit tricky.

I hope that it would help somebody. (because someone like me who are not that familiar to odoo development, it's not easy to fully enjoy the power of odoo. I feel odoo/oca have to provide more/better documentation.)

Let me explain what I did step by step:

1. Create a new class which inherit from "sale_order" (I found quotation is not a class but alias of sale_order - named "sale.order" when it is in certain states.)

file name: sales_extension/sale_order.py

from openerp import fields, models, api

class sale_order(models.Model):
    '''
    extension to existing sale.order model
    '''
    _inherit = 'sale.order'
    
    state = fields.Selection(selection_add=[('quotation_approved', "Quotation Approved")])
    
    @api.one
    def action_quotation_approve(self):
        self.state = 'quotation_approved'

2. Define a form view to show "Approve" button in front of "Send by Email" button. It only appears when the quotation is in draft state. And to prevent sending or printing quotation which is not approved change visibility of original buttons.

file name: sales_extension/sale_order.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
                 
        <record id="my_quotation_form" model="ir.ui.view">
            <field name="name">sale.order.form</field>
            <field name="model">sale.order</field>
            <field name="inherit_id" ref="sale.view_order_form"/>
            <field name="arch" type="xml">
                    <button name="action_quotation_send" position="before">
                        <button name="approve_quotation" string="Approve" states="draft"
                            type="workflow" class="oe_highlight" groups="base.group_user"/>
                    </button>
                    <button name="action_quotation_send" position="attributes">
                        <attribute name="states">quotation_approved,sent,progress,manual</attribute>
                    </button>
                    <button name="print_quotation" position="attributes">
                        <attribute name="states">quotation_approved,sent,progress,manual</attribute>
                    </button>
                    <field name="state" position="attributes">
                        <attribute name="statusbar_visible">draft,quotation_approved,sent,progress,done</attribute>
                    </field>  
            </field>
        </record>
        <record id="my_action_quotations" model="ir.actions.act_window">
            <field name="name">My Quotations</field>
            <field name="type">ir.actions.act_window</field>
            <field name="res_model">sale.order</field>
            <field name="view_type">form</field>
            <field name="view_id" ref="sale.view_quotation_tree"/>
            <field name="view_mode">tree,form,calendar,graph</field>
            <field name="context">{'search_default_my_sale_orders_filter': 1}</field>
            <field name="domain">[('state','in',('draft','quotation_approved', 'sent','cancel'))]</field>
            <field name="search_view_id" ref="sale.view_sales_order_filter"/>
            <field name="help" type="html">
              <p class="oe_view_nocontent_create">
                Click to create a quotation, the first step of a new sale.
              </p><p>
                Odoo will help you handle efficiently the complete sale flow:
                from the quotation to the sales order, the
                delivery, the invoicing and the payment collection.
              </p><p>
                The social feature helps you organize discussions on each sales
                order, and allow your customers to keep track of the evolution
                of the sales order.
              </p>
            </field>
        </record>

        <menuitem action="my_action_quotations"
            name="My Quotation"
            id="menu_my_quotation"
            parent="base.menu_sales"
            sequence="99"/>
        

    </data>
</openerp>

3. Extend the workflow to add a new transition "draft" -> "qutation_approved". I'm not redefining workflow but referencing existing workflow of which the ID: wkf_sale, Name: sale.order.basic. I found it in "sale_workflow.xml" file under "sale" module.

file: sales_extension/sale_order_workflow.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="act_quotation_approved" model="workflow.activity">
            <field name="wkf_id" ref="sale.wkf_sale"/>
            <field name="name">Approved</field>
            <field name="kind">function</field>
            <field name="action">action_quotation_approve()</field>
        </record>
        <record id="trans_quotation_draft_to_approved" model="workflow.transition">
            <field name="act_from" ref="sale.act_draft"/>
            <field name="act_to" ref="act_quotation_approved"/>
            <field name="signal">approve_quotation</field>
        </record>
    </data>
</openerp>

4. The last step: manifest file and init file.

file: sales_extension/__openerp__.py

{
    "name": "My Sales",
    "version": "1.0",
    "category": "Sales Management",
    "depends": ['sale'],
    "description": """
    This is for demo extension to sale order
    """,
    'demo': [],
    'data': [
             'sale_order.xml',
             'sale_order_workflow.xml'
    ],
    'test': [],
    'author': "Kibong",
    'installable': True,
    'auto_install': False,
}

file: sales_extension/__init__.py

# -*- coding: utf-8 -*-
from openerp import http
import sale_order

=======================

I'm a lazy person so I really hate reinventing wheels. But you can see I had to copy existing  <record id="my_action_quotations" model="ir.actions.act_window"> part from original source and modified it to add behavior linked to new state. Anyone can give idea how to extend not to rewrite it?