Rall,
If you just download the PM software you can test if it will work with the other software you use. Even if i doub it because PMDG is a complete own writen software and PM works with the FSCuip of peter Downson.
You also cant use any freeware prog because to get PM PDF/ND working you need to buy it so what would be than the point to use any freeware software if you already have PM.
I assum if you know the keysend of your software you use for the typical plane you can change the CDU keysend as well i am not quit sure how but i guess that Peter Cos can tell you or just will help you to change the basic PM keysend to any other way you want to use to send it.
Succes
Frans M
www.airbus320.nl