Stored Procedure with override?

I received a request to create a stored proc with following:I have a view with the following columns(table1 - AppCode, AgencyID, CompnyID, CustCode, CustVal)I have a proc that will take the following parameters and return all matching rows (AppCode, AgencyID, CompnyID, CustCode(optional))The trick: Any customcode with the CompnyID should override the AgencyID parameter.Can anyone point me in the right direction with this? Any help would be greatly appreciated. Thx!