Right now, Moviesandbox only supports COLLADA import, and also only spotty at best - given that COLLADA is a bit of a mess of a format. The only program I was able to successfully export from was 3DStudio Max, but I will take a look at other programs mentioned to see if I can fix things up a bit.
As for characters, unfortunately you will only be able to modify characters that you created within moviesandbox. There's no general mesh editing, and Moviesandbox's mesh structure is particle based, so it's significantly different.
The idea of regular mesh editing is noted and I'll at some point get to implement that too, but it'll take a while.
With that out of the way, welcome to the forums!