[CentOS] Importing md5 encrypted passwords for el6

In the process of replacing an el5 server, I have
installed el6 and scripted the creation of the users.
That worked flawlessly.  I then imported all the md5
encrypted passwords, and users cannot log in.  I see
that the new default encryption is sha512, but I
thought that only applied to creating new passwords
and not the decryption of existing passwords.

A couple of hours of reading man pages and Internet
searches has left me clueless.  Is there a way to
keep sha512 as the default and to enable usage of
md5 in addition to sha512?

C