To add a new field to an existing module, the best practice is to create a new module that inherits the model you want this field to add in. And your module will depend on the required module.

For example adding field cartage to account.invoice model, I created a module called invoice_cartage:

inheritance.py:

from osv import fields, osv
class invoice_cartage(osv.osv):
    _name='account.invoice'
    _inherit='account.invoice'
    _columns={
               'cartage':fields.float('Cartage',readonly=False),       
              }
invoice_cartage()

inheritance_view.xml:

<?xml version="1.0" encoding="UTF-8"?>
     <openerp>
         <data>
              <record id="invoice_form" model="ir.ui.view">
                     <field name="name">account.invoice.form.inherit</field>
                     <field name="model">account.invoice</field>
                     <field name="inherit_id" ref="account.invoice_form" />
                     <field name="arch" type="xml">
                     <data>
                            <field name="labour" position="after">
                            <field name="cartage"/>
                            </field>
                     </data>
                     </field>
             </record>
             <record id="invoice_tree" model="ir.ui.view">
                     <field name="name">account.invoice.tree.inherit</field>
                     <field name="model">account.invoice</field>
                     <field name="inherit_id" ref="account.invoice_tree" />
                     <field name="arch" type="xml">
                     <data>
                          <field name="amount_untaxed" position="after">
                          <field name="cartage" sum="Cartage Amount"/>
                          </field>
                     </data>
                     </field>
             </record>
        </data>
</openerp>

Use ref attribute whenever using inheritance. This is the view id of the view to inherit from.

Note position attribute. You need to mention where you want to place this field on the view, whether after a field or before. Set postion=after/before accordingly with the field of the base view.

Thats all.