Yes. Microsoft has given the message, constantly, that they would accept pull requests. The workflow is still being worked out but many have contributed to many of the .Net components they have opened sourced.

Right now I think it's a semi manual process to bring code in house and semi automatic when it gets pushed into GitHub. At least that's what one of the last Microsoft engineers described when they were on HN a few months ago.

of course not. Embrace, extend, extinguish is all they want.