download the shadow 4.03 package, it has all the login and passwd tools, modify the passwd.c file and the other files it uses as needed and recompile them, then use that
this isnt easy, my C programming skills suck, wish I could help more
you should be able to use an unshadowed passwd for the users but that is a security risk, see the passwd or shadow how-to at
http://www.tldp.org