I've been working with DNN since late 2002 when it was first released, and developing software professionally since 1979
The passwords can be stored as plain text, however the MD5, and other hash algorithms are one way functions, so the passwords can not be "un-hashed" and converted to plain text.
One solution would be to store any new or changed passwords in plain text, and have the authentication routine try the hash algorithm first, and then try to authenticate the user's password as plain text, if that fails then the authentication would fail. An email would need to be sent to users instructing them to change their passwords.
Another solution would be to change all the user's passwords, storing them as plain text, and send them all emails with the new password, not recommended due to security issues.
How many passwords do you have?
Please contact me on freelancer to discuss this further, there may be other options