Modifying standard SAP screen and PAI/PBO modules?

Hi,

I have a SAP screen in which we need to dipplay and populate a Z-field. For this particular screen I haven't found any suitable user-exit or BAdi, so what are my options for moifying the screen? Is there some modification flag or modification assistant in ABAP Screen Painter that can be used so that future SAP support packages and additions will detect the change made to the screen (so that the modification isn't overwritten in future releases)?

Thanks for your help. Points will be given!

Regards,

/ Christian