As stated on the Keil Forum:

The easy and safe method (I assume you will use assembler subroutines in a "C" main) is to write a 'skeleton' routine in "C" and compile it with the SRC directive. Then cut out the assembly code. Then cut the object with a coloumn edit. Now you have the entry and exit for your assembler routine and all interface questions are answered, so all there is left is to code the guts of the routine.

Erik