[dcb]

EVs can be configured to perform "regenerative braking", which is essentially what you are describing with the generator head. Most motors can also be used as generators and when you want to slow your EV down, the vehicles momentum is used to charge the batteries through the traction motor (now being used as a generator).

Electrical regenerative braking does help a little bit, especially if you have to brake a lot. On a long hiway cruise, not so much. But it is bound to be more efficient (and thus likely to help) if you use the EV existing motor/generator than adding a separate generator head and wheel.

You can also add battery packs to improve the range of an EV, though it isn't one for one, doubling the battery pack size will increase weight which reduces the expected range from the additional capacity.