This is a great idea and sounds like it would be a good community project if enough people were interested in dedicating so time toward such a thing. I loved those books.
I really think that D&D could have learned something from the way that the kai warrior gained new abilities. The only class in D&D that follows this model now is the fighter, who can choose his new powers as he increases in level. All of the non-spellcaster classes should have followed this model.
The rogue has hints of it at higher levels when he gets to choose special abilities, but he should have been able to select all of his abilities, like sneak attack, evasion, etc.
The Kai Warrior could be something like this:
Hit Die: d8 (as monk)
Skills per level: 4 per level (as monk but new skill list)
BaB: medium (as monk)
saves: all saves good progression (as monk)
class abilities:
levels 1-10: Choose a kai power
levels 11-20: Choose a magnakai power
The powers could be adapted from the powers in the books. If they are balanced properly then this class should compare very well relative to the monk. In fact, I think that in some ways, this would kind of be like what the monk SHOULD have been, instead of the monk we have now with fixed abilities that can't be customized. Perhaps I'll do a complete write up later as I'm kind of inspired now. The Kai Warrior would make a really excellent D&D class.
I think the hard part would be converting the powers.