PL/C functions

Is it possible to write functions for PL/C that have variable argument lists?

I.E.
UPPER( arg )
UPPER( arg, encoding_list )

LOWER( arg )
LOWER( arg, encoding_list )

SUBSTR( arg, arg, arg )
SUBSTR( arg, arg, arg, encoding_list );

Obviously, these would have to take the place of the original functions, is that possible?