If a login is already created for that user, you can use:
SELECT name, sid FROM master..sysxlogins WHERE (xstatus & 4) = 4
The tricky part would be now to convert the varbinary value (which looks like this: 0x010500000000000515000000B644E4233B245E6907E53B2BEB030000) to a more readable thing (e.g. "S-1-5-21-602162358-1767777339-725345543-1003").
Razvan