Hi,
Unfortunately it's the wrong program. A feature I'd like as well but not something Music Maker is for. It's for recording and arranging songs with recordings or samples. You can't create chords or modify an existing chord to make it a minor.

The nature of a major chord vs a minor is simply 1 or 2 different notes. You'd have to be able to change those notes in an existing chord to change it... not possible with any application.

That being said... you can accomplish this via MIDI and that might be your best option. MM15 is ok with MIDI but again, not the right program there either.

I hope this helps,
Justin