I don't know if this is baked or not, but it should be.
The idea is for an electric wheelchair that you can propel yourself using your own energy, or you can use the electric system.
It would need to have the same type of wheels as self propelled wheelchairs or similar with a hand grip to propel
them self. The wheels might also need to be slightly wider than normal self propelled wheelchair wheels so that the chair will have enough grip to accommodate for the larger amount of weight.
By propelling the chair your self, the battery power will last much longer and the user will get much more exercise than with an only electric wheelchair.
The wheelchair might need some type of clutches to disengage the motors from the drive wheels, to make it easier to propel the chair. Or if you really wanted to exercise, you could leave the motors engaged and maybe even help charge the batteries while your at it.
The chair might even be able to have regenerative braking, but it probably wouldn't be very useful, because you probably would never be traveling fast enough or be going down a steep enough hill long enough, it might help some, but not for most applications.
The control system could be programmed to stop the wheelchair from rolling backwards down an incline while the user is trying to propel them self up it.
The hand grips could be attached to the wheels with some kind of planetary gearing system (I don't have time to figure out how it would work right now) so that the user will have a gear reduction to the wheels since the chair is heavier, which would also make it slower.