You're not going to pair Greinke with Strasburg by trading pitching only. Greinke is the only pitcher you have that will get you Strasburg in return. If you want to pair Greinke with Strasburg, you'll have to put together a trade package that involves a good bat. I don't know what your league settings are, but 9 times out of 10, IMO, bats >>> pitching.
OF- Myers Jennings Heyward
U- Morneau, Ellsbury, Belt
SP-Price Greinke Fernandez Lee Miller
RP-Nathan, Grilli, Farquhar, Melacon