I typically find it easier to manage Custom Attributes by Role mapping instead of User mapping because users that share common attributes tend to be in the the same groups like shop users or shop supervisors etc.
If you have access to the Netweaver server's database you may be able to update the attribute tables directly. The tables are XMII_CSTMATTRIB and XMII_CSTMATTRIBMAP. This method can be helpful for an initial data load but I wouldn't recomend it for a regular business process.
In practice I like to store user data in custom tables or MDO objects. That way I can give the business users / administrators like managers and hr etc and easy way to manage the data vs the attribute configuration screens.
Regards,
Christian