I guess magic alternators are your only hope. Simply said, you cannot get more energy out than you put in.
Say your car motor is outputting 15kW in mechanical power, so accouting for losses and imperfections you need for example 16kW from the batteries. Now, lets say you load the motor with an alternator, thus add 5kW load on top the normal load form the ride. So now you need to draw 21.3kW from the battery. Using the same losses for the alternator, it provides 4.7kW to recharge the batteries. Sum it up, 21.3-4.7=16.6kW net load on the batteries. Whoops, adding the alternator-charger cycle, you are all of sudden wasting 1600W more than when you started, and this is not accounting for the additional weight of the alternator and charger etc.
See my point? Losses, losses, losses, and there is no way around that apart from magic.