only if you write a logon module, that does the cropping and changes the username from user@realm back to user. If you want to do this, I can dig whether I can find an example logon module in my archives. However you should be able to just do a mapping from ABAP username to LDAP username (given that the usernames are equal on both realms) which then would do the same trick.
If you usernames are however different on the both realms, you will however have to have some other mechanism.
Regards,
Patirck