can a battery powered motor act as a generator when faced with oposite resistance?

Yep. If you turn the axial on the motor it will create an electric current. You can see this if you attach it to a multimeter, although will probably have to spin it extremely fast and have the multimeter on a low setting for it the current to be picked up by the multimeter.

I don't think you would get enough current to recharge the batteries very much though. You could try and see what happens. Make sure you get the polarity right or you will uncharge them.

Also i think you need a higher voltage to recharge the batteries, I am not sure though.

It is possible to set up a system like that, but I don't think any current RC cars would work that way. First, you need a type of motor that supports it. Easy enough.

The harder part is the control circuitry of the car needs to know how to regulate the voltage from motor/generator in order to charge the batteries. Most likely the rectifiers in the car won't allow the current from the wheels to get back to the battery. You don't want it to unless it is at the correct charging voltage of the battery which would require some sort of voltage regulator control in the motor/generator.

Actually, the system you're suggesting is similar to what is done with regenerative breaking in hybrid and electric cars.

A motor and a generator are in principle the same device. So yes in theory you could, but in practice it likely will not yield enough current to charge the batteries.

Sounds like a really neat science fair experiment for someone to try.

Yes it works. But it will consume alot of time to charge the batteries.