We have the same problem at one ISP. We have even have to store different encrypted passwords (eg. Irix, Solaris, Netscape ...) into an LDAP directory to be able to authenticate all of them using this LDAP server. And for chap, we needed to store the plaintext too.
Probably the easiest move is to store plaintext in there, if you have them. (It makes sense to store the plaintext password (or you at least have the possibility to decrypt it) in a billing system. Than you can always switch from system A to B.)