>Unfortunately I cannot change the procedure code and I have to use DO PROCEDURE because, in my case, I have to specify the IN clause to tell it where my procedure is. If anyone can help me out I would appreciate it.

I think your main Problem here is the IN clause, since this isn't honoured by function calls.
Use Mike Yeager's approach and create wrappers in the app, where x and y are defined:

Procedure WrapX(tlReturn)
tlreturn = x()

Procedure WrapY(tlReturn)
tlreturn = y()

which can be called like

Do WrapX in MyAppToCall with llMyXReturnValue

from your normal code. It's a bit safer (and minimally slower) than the set proc approach...

HTH

thomas