Apple certainly could, but there's a thread running today on that very topic in which a company says they intend to do that. That's what I thought you were talking about. I don't think Apple will, because batteries produce heat on charging and discharging. . . Not something I would want wrapped around my wrist. ;^(
The batteries will be in the watch, The band will hold the equipment needed to touch the skin in order to get the body voltage and/or kinetic movement, and transfer the energy into the batteries in the watch.
THe batteries themselves in the next version of the watch will be more advanced nanotechnology designed batteries that are currently under development. THey will charge far faster and will not generate the heat issues, the way current ones do.
For gen 1 users they’ll need to get the new band, and probably have to exchange their current batteries for the new next-gen batteries that will work optimally with the new charging band.