Suggestion for Mapping logic needed

Hi

I have a condition were in if a field value is A1 i should pass it as 01 to target and if its A2 i should pass it as 02 to target side..

We can do it using fixed valued fn..But if the field value is anything othet than A1 or A2 what ever comes it has to be passed over unchanged..

How to get it..Do we need a UDF or standrd fns can do it?