Well, you have a couple of problems with the question.
First of all, ABC.DEF is not a valid name for a cataloged procedure.
You haven't told us how you pass this X to the ADUUMAIN utility. If it is sent to the utility through a control statement in an input dataset ADUUMAIN reads there is nothing in JCL you do directly. If it is passed as a parameter, like EXEC PGM=ADUUMAIN,PARM='FUNCTION=X' you have a couple of options. The simplrst would be to define the procedure like this -
//ABCDEF PROC FUNCTN='?'
//A EXEC PGM=ADUUMAIN,PARM='FUNCTION=&FUNCTN'
and call the procedure like this -
//A EXEC ABCDEF,FUNCTN='X'
There are other ways to achieve this effect, but this is the most traditional.