If I understand you correctly, the imported users are placed into a group (otherwise, I can’t imagine having a private category).
You could make the group visible only to group owners/members (in effect, being invisible to everyone who is not in the group). This is done in group settings.
I don’t think anything can prevent other users from seeing a user profile, but if the group is private, they won’t see who’s in that group. They won’t be able to tell if someone is in a private group unless that someone has a title specific to that group, but then again, they won’t know anything beyond that.