If you don't know how to re-program the MCU, then no, you cannot add more servos to the MCU without replacing the MCU.
If you don't know how to hook into the receiver output, then no, you cannot use the existing remote control to control new servos.
"toy car" is impossibly wide to talk sensibly about, though. A $29 Radio Shack RC special is very different from a $499 kit car that's made of individually replaceable components.
What toy car is it, and did it come with any instructions, and/or is it modular in any way?