UME_ABAP: How to manage additional SU01 values

EP 7.01 with ABAP (ECC 6.03) user persistence store...

Creating users in Portal UME automatically creates user in ABAP backend. How do I get the "User group" field on the "Logon data" tab (when viewed in SU01) to be editable in Portal UME? This field is stored in backend in USR02-CLASS.