How to use main program for engraving letters from A to Z and numbers from zero to nine

I have to use one main program for engraving letters from A to Z and numbers from zero to nine.

I need to select any one alphabet randomly and any one number randomly to create any word  maybe 12-15 letters ( including repeated alphabets as i desire e.g  A LL E P E )  . I have separate subprograms created for each of these engraving letters and numbers. In all 36 subprograms are available for engraving each letters or numbers in CMA folder .

How can i define them as variable in GUD and then use them in my mpf file to be executed for engraving.

Currently i am calling each required subprogram in my mpf file & using them  as required  . I want to use them in customised screen .

Kindly guide.

------------------------------------------------------------------------------------------
Split from GUD variables problem (APWB).