Awesome topic. Research has shown that fast twitch fibers can convert to slow twitch, but slow twitch fibers have much difficulty converting to fast twitch, hence part of the reason why sprinters can be good at endurance but endurance runners can't sprint...
Only a small piece of the picture though. Fast twitch fibers are stimulated by anaerobic activity. Baseball is 99% anaerobic. Like OK said, sprints, heavy lifting with lop reps, and other short burst activities stimulate II b fibers. Hopefully that helps a little bit.