What you are suggesting is not really possible the WAY you are suggesting it. It sounds to me like what you want is wireless digital communication. Why not simply use existing devices? You might consider using a wireless modem in your "car" and controlling it via IP from a PC. Proper selection of a uController should allow using open source code for >90% of the USB to Modem interface, and there is plenty of open source code for handling general IP packet handling. From there you would just need to build servo driver interfaces from the uController to your servos.
Otherwise, as previously suggested, the best answer would be to replace the human interfaces in your transmitter with DACs (Digital to Analog Converters). The theory here is that you have an existing radio interface between the transmitter and the car that works, if all you want to do is control the car from a PC, then simply replace the "joy sticks" with "Computer Interfaces" (DACs).