Migrate from older passwd/shadow to NIS
 

I'm trying to upgrade a network (1 server/10 clients) which have an older version Slackware to RH 7.2. I thought it would be a good idea to try and install NIS (now the etc[including passwd and shadow] is synchronized periodically with rdist).
The problem is how do I add all the users to NIS (about 200 of them) without changing all the passwords (it would be murder, mostly for the users, not for me).
Another thing would be how do I import the present passwd/shadow into RH 7.2 (the present version dates from 1998 and I'm very sure it doesn't have MD5 or any other funny business).
Thanks all for reading.