Yes it is possible. However, there is more to it than just making some of the units playable, there are lots of issues relating to MP that don't crop up in SP. If you are not familiar with the differences between client side and server side effects then read some of the tutes in the Ed Depot.
However, un pbo-ing the missions and opening them up in the Editor shouldn't be a problem. I use Amalfi's pbo decryptor 1.5 available in the Editors Depot and it works fine.