No.... This will never be done.
1) Messenger Plus! is made only for Windows/MSN Messenger, and that is already taking a full time job. Including another protocol would be too much.
2) Proper protocol handling should be done by the client. Messenger Plus! isn't a client on itself, but an addon for a client.
3) Messenger Plus! doesn't do anything with any protocol (including the Messenger protocol)
What you could do is using a client which supports both protocols, like
Trillian, but then you would missout on the great Messenger Plus! features of course....
/ 
