This is the process I run. There may be better ways...
On the old system, create a copy of the existing /etc/passwd and /etc/shadow files.
From those files, remove all the "SYSTEM" accounts. Also remove any users which will not be migrated to the new system. When you are done, both files should contain the same number of lines.
Using one of those files as a guide, create a tarfile of the affected users' $HOME directories.
Copy that tarfile and the temporary passwd and shadow files to the new box.
Restore the $HOME tarfile. Since these are new users, you shouldn't be over-writing anything. If in doubt, check first.
Append the transferred passwd and shadow files to the existing entries in /etc: (note use of APPEND (>>) syntax!!!!)
# cat old.passwd >>/etc/passwd
# cat old.shadow >>/etc/shadow
Run the TCB utility to syncronize the protected password database:
# /tcb/bin/authck -p
"Proof that there is intelligent life in Oregon. Well, Life anyway.