Hi Laura,
I understand that you are trying to make the Priority field (PRIOK) in maintenance item mandatory. It is a very simple task with User-Exit. Just put the following code in the include ZXPRMU06 of user-exit IPRM0004.
IF IMPOS_IMP-PRIOK IS INITIAL. MESSAGE: 'Please Fill the Priority field.' TYPE 'I'. RAISE NO_UPDATE. ENDIF.
You will get this message and the Plan is refused to be saved until Priority Field is filled.
Your ABAPer will be able to do it in few seconds.
Jogeswara Rao K
PS: Also for me SHD0 option has always been last resort.