With the help of k0Do from #xfce i could solve the problem, quick and easy.
I installed the genmon-plugin
and let it run a simple bash script(echo $USER)
and thats it, the username is displayed in the panel. I will put the script in a
path readable for all users and edit the default panel file etc. to include it
for every new user. I also found out about the xfce kiosk mode, so i will lockup
xfce, so no user could delete the plugins etc.
Thx to k0Do and the posters in this thread.