Hi Markus,
if you use RSA1, you need to schedule a process chain every 5 minute or so, which uploads ZEXTERN attrbutes and texts to ZDUMMY. If you do it your self, do don't need to schedule it but have to think of executing the process chain after you saved the value in ZEXTERN.
Bi-IP or BO Analysis with planning function would be better because then you can call the process chain immediately when you press the "SAVE" button in the application.
Regards,
Jürgen