If you have your MRE repository in an RDBMS you can use SQL to update user information. I normally recommend the use of the GUI for maintaining role, group and domain information since this doesn't change often then use SQL to add a row for the user in WF_MRUSERS and then one row each for the user to group relationships. the WF_USRGRPTB.
There is no supported migration routine to migrate from the file based repository to a RDBMS repository. Send me an email and I can send you something which was field developed if you would like to migrate and use SQL. ron_eernisse@ibi.com