Arcgis 10.1 modelbuilder - defining a field name in a expression to be a parameter

Hello,

I was asked if the following can be done in the modelBuilder (I know it's simple in python, but I was asked specifically on the modelBuilder):

The user has a expression in the model: str(!city_code_int!).zfill(4) (the "city_code_int" is the field name of an "long"-type field).

he wants to put only the field name "city_code_int" as a parameter in the model (and not the entire expression).

Is it possible to do such a thing in the 10.1 Model Builder?

Thanks in Advance.