Kelly wants to know if it is possible to paint over something that already has stain on it.
The answer is YES, but only if the stain is ready! What do I mean by that?
Most stain has water repellents built into it. Look at the first photo and you can see how the water is beading up, rather than soaking into the deck. If you try to paint over stain that is effectively preventing penetration, the paint won't stick well. The second photo shows a board where we put water repellent on one half, and not on the other half. When you wipe on a paint or a stain, it just doesn't penetrate on the one side, as you can see when I wipe it off while wet.
Let any water repellent weather until it allows water to soak into the surface before trying to paint over it.