Allegro is one of the Coda Music programs, along with Finale and PrintMusic, and is the "bargain" program they recommend for "midi-intensive" work. It appears that all of their programs use the .mus file spec. Since Coda is one of the "principals" in the business, you may find other (competing) products that will import .mus files, but probably not many that will export them. (Their cheapest program, Finale Notepad, is so limited I wouldn't bother with it. It doesn't do midi(?).)
As far as I can tell, none of the common commercial programs aknowledges the existence of ABC. There may be one out there that will import an abc file, but I haven't seen that mentioned in any of their descriptions.
AbcPlay Also at: AbcPlay Home An ABC player for a PC Windows system with a Midi/Sound card. The program has a number of handy features to make to hearing of ABC files easy, also has cut/paste functions and the ability to save tunes as a Midi File. Written by Bernard Chenery, Tasmania.
abcMIDI Also at: abcMIDI Home James Allwright's package to convert abc to MIDI and MIDI to abc. Since most standard music packages support MIDI output, this should make it easier to generate abc from commercial software. It is available on the web as C source code. It also contains abc2abc which can be used to transpose abc.
and
abcMIDIfier Also at abcMIDIfier Home A Windows utility by Dave Glenn which provides a user-friendly front-end to James Allwright's abcMIDI package.
Perhaps one of these would be helpful. They should all be shareware, and may be available as trial copies - I haven't checked.
Pavane's program (or one of his programs) has been reported as doing a good job of converting abc to midi. I don't have a site url handy, but if you find any thread he has posted to and click his name, it should give you plenty of places to find him - if he doesn't get here first.
The suggestion would be that you make a midi from the abc and then almost any notation program will import the midi and turn it into a score.