As for a z/OS "main menu", there are really two possibilities: the VTAM menu previously mentioned, or a selection menu provided by a session manager program. The VTAM menu must be updated by VTAM support people; you may (depending on the session manger program) be able to influence the session manger selection screen yourself. I used the CA session manager for many years, and it allowed you to update the selection screen, though it was not all that easy.
You can alter the ISPF PRIMARY OPTIONS menu yourself. It's member ISR@PRIM in one the data sets in the ISPPLIB concatenation. What you do is allocate your own PDS, copy the standard system ISR@PRIM member to your PDS, update it as you please (that's the tricky part, ISPF is not the most friendly environment at times), insert your data set into the ISPPLIB concatenation while your TSO session is in the READY prompt, and start ISPF like you normally start it.