BESM is a poor system IMO. The stats barely mean anything since you wind up adding all three to 90% of your rolls in the first place, the abilities you buy with your points vary wildly in power per point, and infinite loops are extremely common. Whats more, many of the abilities are so open-ended that you just about have to make your character with your GM over your shoulder telling you what you can and can't do with it. Finally, I failed to find rules for any sort of character progression whatsoever.
If you still want to do this, I suggest mimiccing God of War. I played God of War 2 and it actually had a very nice, if simplistic, method of character advancement that I am uncertain whether or not was also in the other 3 games. You collected and sacrificed special items in order to increase your heath and magic power and your XP was used to power up abilities originally granted through story progression. If you add ability scores and derived values to that then you've got the beginnings of a good system.
Whether or not someone has actually done this... I have no clue. Now that I think about it, I just might do it myself.