I am writting a new module which inherits account.invoice adding a new field.

I would like to add an action to populate that new field under an onchange_partner_id event.

However, I would not like to rewrite/replace the whole routine, just do some extra stuff at the end.

Is that possible: to add a second onchange event to be executed after the main class one or in parallel to the main class?

If so, how?