this is done with Sony vegas.
I am still using vegas 6.0a (Build99), because it works perfectly I only update, when there is a reason to do that. Lots of new version work worse than the old ones.
Best example for freeware: besweet
Anyway, back to Sony Vegas:
File --> Import --> Media... (choose your 2.0 track, best would me PCM)
Now it will check for peaks. After that:
File --> Render As... (select 5.1 surround sound under templates) --> save
there you go and have a fake 5.1 ac-3 which sounds much much better than a 2.0 track, especially on surround systems.