No, you can't edit individual parts of a kit..you can only change kits, and edit the whole kit.

I believe you can edit individual drums on the Roland G70 and E-series....I don't know about Korg.

You do have two drum tracks on the Yamaha, so you could essentially have a snare or other drum, from one kit and use it with another (after erasing the first), and that track containing that particular sound, could be edited for panning, filter/brightness and effects...it is essentially a workaround, and not nearly as good as Roland's system.

I'm just surprised that if the market for an arranger for the younger player is as healthy as you describe, that no one has tapped into it...surely they must be aware?