I'm trying to import over 5000 users into drupal by using Feeds module. Can I import user "id" into "uid"? If so, how can I do that? I saw the GUID mapping target, but that is not "uid". Thanks.
No, the UID field on Drupal users is auto-increment, so you don't want to import your ID values into that field. It's much better to add an oringial_id field to retain your legacy user IDs.