I am a US citizen and got married in Feb 2014. Filed for my wife's immigration and expect it to go through in June 2015. I'm planning to file an extension and then file a Joint return later when she obtains a SSN. Can I file my extension as Single and later file a MFJ return?

If you are married, you must file as married filing separately or jointly, never single!

If your alien spouse does not (and cannot) get a SSN, you get an ITIN from the IRS: Individual Taxpayer Identification Number (ITIN)

Good luck