Here's the deal, only one study (that I know of) demonstrates an increase in the Type IIB(X) population. This study was performed with short, high intensity bouts of sprints. Here's the reference:
Jansson E, Esbjornsson M, Holm I, et al. (1990) Increase in the proportion of fast-twitch muscle fibers by sprint training in males. Acta Physiol Scand; 140: 359-63.
Unfortunately, no one has been able to replicate the results - and that's not good.
A general statement can be made regarding fiber types: the more they're recruited, the more endurance characteristics they'll exhibit. I hate to make that statement, but based on what's been published (up to this point), that's the case.
Actually, it makes sense, if you think about it. When we recruit a certain pool of MUs on a consistent basis, we're telling our body that they're important for our needs. As such, the body responds by upregulating metabolic qualities that allow for faster recovery (ie, more oxidative characteristics).
But, I'm not completely convinced that we can't increase our type IIB/X pool since many power athletes demonstrate huge populations of type IIB/X fibers (relative to normal). The question is: Were they born that way, or did their training enhance the type IIB/X fibers. Since NIH doesn't consider the answer integral to curing disease, no one knows.