I think yes.

Example on_change method:. link text

Just add any field for output in your result.

def custom_function(self, cr, uid, ids,  field_name_from_xml,  context=None):
    # work with input data
    #... your result in custom_addr
    custom_addr = 'Barcelona'
    return {
        'value': {
            'address': custom_addr # adress is must in your xml declaration in this case
        }
}