[root@MRB home]# su 10aielmi
bash: /mnt/sdb/home/10aielmi//.bashrc: Permission denied
You need to give the user not only the same name as the old one but the same user ID, too. That's the number in the 3rd field of the /etc/passwd line for the user. Or you could chown the files.
If it was my personal system (where I don't mind breakage to learn things) I'd copy the lines for those 30 users from the old passwd and shadow files to the new one ... take care (make sure you can always get back to where you started).
You might have similar issues with group names/numbers. Useful to create any new groups with same numbers as old system if copying files from old system.