On older Electric steering systems the steering computer was a separate unit. All you would need to do is make sure you have that unit communicating with the correct sensors... Find out what value's are needed for the steering to work properly and then use resistor's or amplifiers to limit or add current.
What car is the system you have out of...
Do a google search on how the electric steering works... I've thought about this before and I ever track down a system that is contained in the rack its self and just needs a ( steering Sensor ) to operate correctly I'm going to attempt it.