There may be a few ways to do what your asking. The hard way is get an external drive, drag all your music onto it and transfer all the music under the iTunes account you want to make iTunes Match ( I'd choose the account you purchased content with, there is less issues if any). Then pursue with iTunes Match
Another way would be linking your two libraries from the options tabs and you can wirelessly transfer music from one pc to another. That might cause account I'd issues when trying to play music on the combined library. I have never done that so your going to have to research that way.
You are right iTunes Match only works per I'd. So you would have to purchase multiple match accounts if you do not consolidate all your music.
Another tip when you do start your iTunes Match upload. If the song will not match, you may have to create a AAC version, then try matching it again.
Out of everything I would suggest the external drive so you can make sure everything gets over correctly.