I am assuming that these are AC sinusoidal motors and not brushless DC (BLDC)?
If they are, the resolvers are used for commutation.
You probabally have two choices if you want to add encoders.
You will still need commutation so the simplest method is to add encoders to the motor shaft.
Or you can fit an encoder that has hall effect track for start up, and then the controller uses the encoder for sinusoidal commutation.
This all comes down to what type or make of drive are you going to use to drive the motors.
Many sophisticated drives that accept resolver input for commutation, will extract a pseudo encoder from the resolver signal, often at programmable resolution.
It is no mean task to fit a commutation encoder and align it to the respective poles.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.