You can go to the menu /Settings/Technical/Database Structure/Models

Search for your module and add a field to the list, the field name need to be saved with prefix 'x_' because it's a custom one. Next you may need to add it to a view, search for the needed one at menu /Settings/Technical/User Interface/Views.

All of this need to be done using the group "Technical Features" to be able to see those menus

For exemple insert a new field in product.template

class product_template(osv.osv):

    _name = 'product.template'

    _inherit = 'product.template'



    _columns = {

       'my_field_id': fields.char('Title"',size=64),

and in the view

   <record id="product_template_form_view" model="ir.ui.view">

        <field name="name">product.template.common.form</field>

        <field name="model">product.template</field>

        <field name="inherit_id" ref="product.product_template_form_view" />

        <field eval="25" name="priority"/>

        <field name="arch" type="xml">

            <field name="uom_id" position="after">

                <field name="my_field_id"/>

            </field>

		</field>

    </record>