Is it possible to edit tabs you've posted and add things to them? Because I have just finished tabbing one of the guitar parts for a song, and I'm wondering if i can add the other guitar part later if I post the tab now.
Yeah you can edit tabs.

In your profile, go to my contributions, select the tab "tabs" and it should give a list of all the tabs you have submitted. in therem the should be a yellow(iirc) box with a u! that you can click to update. However, even if your tab is already accepted, it will have to be reviewed again if you want to make a change to it.