Hello,

The code that opening the opportunities from the button is defined here, you can see that the button calls an action with name:

%(crm.crm_case_category_act_oppor11)d

which you can find it here , the automatic population of the data comes from the context field:

1) in the button's context you'll find:

 context="{'search_default_partner_id': active_id}" 

this set the default search, so that when you click the button you'll find the search area 'right corner' populated with the partner's name more over if you click create you'll find the partner field populated with this one. [try to delete the search filter and press create, then you will miss the partner].

2) in the action's context you'll find:

 <field name="context">{
                'stage_type': 'opportunity',
                'default_type': 'opportunity',
                'default_user_id': uid
            }
        </field>

you can set a default value in a field by using 'default_your_field': your_value, in the context dict.

You can edit this context to add the default value of your fields.

* Note: you said that you added some fields to res.partner , but I think you've to add your fields to Opportunity's model to be able to but I think you've to add your fields to Opportunity's model to be able to set the default values to them ....

-Also you can populate it by using an on_change function ...

*BTW I didn't down voted your question ...

I hope this will be helpful