Hi Ulrike,
This is just for your information only. I also have somewhat similar requirement which is open in a discussion thread below -
http://scn.sap.com/thread/3677845
You can keep following it or investigate it further and update us.
Also there is a list of user exits already published by Maheswaran KD . ,
http://scn.sap.com/docs/DOC-45085
http://scn.sap.com/thread/3676972
Thanks,
Arijit