It is possible, but you would have to purchase the correct posts, because the threading is different from manufacturer to manufacturer. From my experience, firestone has the same threading for both their pin lock and ball lock.
I have also been able to swap the posts between cornelius branded kegs as well.
If you don't have the spare posts already, it isn't worth it though, as each one is usually around 10 dollars a piece.
I second the above comment about using flared connectors for all your tubing so that you can swap between any disconnect you need. I believe I found an online vendor that has the 1/4'' flared connectors for pretty cheap. I have used well over 100 of these for all my different kegerators, jockey boxes, transferring tubes, air distributors, etc. I also have 80 kegs, a good mix of pin and ball lock, so I need to be able to swap the disconnect for whichever keg I want to use.
edit - I should add that I've done these swaps. My friend had some firestone ball locks and a lot of pin locks, and wanted to get everything pin lock, so he just brought the posts, and we did a direct swap of the posts. I also had a leaky cornelius pin lock due to the posts, and I have spare cornelius fittings, so I just put the ball lock posts on that pin lock keg.