hello Math
Here for you I'm adding new tab in "Quotations" of "sale" module,this is similar to your requirement
Example.py
from openerp import fields, models, api, _
class class_name(models.Model):
_inherit = 'sale.order'
<fields name="..."/>
Example.xml
<record id="view_sale_order_tree" model="ir.ui.view">
<field name="name">view.sale.order.tree</field>
<field name="model">sale.order</field>
<field name="inherit_id" ref="sale.view_order_form"/>
<field name="arch" type="xml">
<xpath expr="//page[@string='Other Information']" position="after">
<page string="My_New_Tab">
<field name='My_new_fields'/>
</page>
</xpath>
</field>
</record>