You can tell Ubuntu to point to that directory, and to use existing profiles and passwords.
The first step is to re-add the users to /etc/password, /etc/groups and /etc/shadow. If your setup is simple you might be able to simply copy the old files to the new box and keep the userid's. If you find this is not possible (because the userids are in use) you would create new users and then change the ownership of the home directories for the users.
There are a number of ways of dealing to the old home directories -
- You could simply move them to /home/USERNAME on the new box
- You create symlinks from /home/USERNAME on the new hard drive to their location
on the old drive.
- You could modify the users home directory as specified in the /etc/password file
to point to the old location.