It is totally feasible to modify a servo with one type of feed back, and adapt it for another. You will need to figure out how many poles your Sanyo Servomotor is- I am assuming it is an AC servo motor. When you know how many poles, you need to find a resolver that will work. Essentially two parameters need to be focused on: Resolver speed (or how many pole resolver) does not have to match that of the motor, however you want to be sure your drive can handle it. If the drive can only handle a 1 speed or 3 speed resolver, and you have a 4 or a 6 then the motor will not run properly. The second and more challenging part is size. The rotor of the resolver has to fit on the shaft. Once you remove the encoder ( and hopefully its not one of those Sanyos where the glass it epoxied to the shaft) you will need to measure the diameter of the shaft and determine if you need to turn it down, or make a sleeve to adjust the diameter of the shaft to accomodate the rotor (Rotor of the resolver I mean). Next you'll need some way to secure the Stator of the resolver - the easiest way is to drill and tap holes for cam-locks. I may be mis-spealing with camlocks, I means spacers or washers with tabs that fit into the groves at the bottom of a resolver. Essentially any way you can figure out to keep the stator fixed in place, and the rotor on the shaft. From there you either have to align the resolver - which can be complicated, or if your drive has an auto-tune funcion you have nothing to worry about. Most retrofits i've done have been replacing encoders with different brand encoders. Hope that was of some use.
P.S. I would look on ebay for resolvers: Harrowe is the brand I've seen most often, Tamagawa are pretty standard too.