Can i add to my return if i have already filed. i have already filed married but filing separately and want to change it to filing jointly. Can i change it even though i have already filed.

You can not change a return once it is filed.  What you can do is file an Amended Return.  Below is the link to the TurboTax info page on this.  In your case you will almost certainly get some more money back from the IRS.