I wouldn't suggest converting a cage induction machine but of course it can be done and it is certainly no more difficult than making a permanent magnet rotor.
You should be able to find a wound field alternator that is a better starting point than converting a cage motor. If you are lucky to find an ancient wound field induction motor then fine it will behave just as a wound field alternator. Just depends on what you can find.
http://johansense.com/bulk/repulsioninduction_1.JPGhttp://johansense.com/bulk/repulsion-induction_2.JPGi happen to have one of these things.
that's a half hp motor, 1725 RPM, about the same size as a 5 HP 3 Phase 3450 RPM motor.
the commutator is arranged with a steel spring inside the cup, which shorts out all the bars until it gets about say 800 rpm, after which the current is passes through carbon brushes. i modified the brush arrangement so the angle can be changed, its fun to play with but that's about it. i suppose i could hook a 60 hz MSW inverter to the rotor, and the line to the stator and see what happens when i try and spin it with VFD..
there is a thread on this forum where i did a bunch of experiments with making a wound rotor synchronous generator from an induction motor.
the biggest problem i ran into was: small induction motors are designed to run with EVERYTHING completely saturated. the shell of the motor helps pass the flux around the core lol, and about half of the flux doesn't make it through the rotor core, it runs through the teeth back to the core. (count the turns and use the standard transformer formulas to find what the flux would be...)
I was able to hit about 40 watts no load rotor loss on the equivalent of a 3 HP 3400 RPM motor core, running it with 4 poles instead, and the core maxed out at about 1.3T or so, but that's with no load.
once you throw a load on it, you have to add that many more amp turns to the rotor, which means in practicality you'd be looking at for a 3-5 HP motor, probably about half of your copper losses are in the rotor.
Unfortunately there is no good way to augment the rotor with magnets, unless you have a milling machine (which i do) and you have a solid block of steel to start over with for the rotor core. (which i don't, and don't know where to get around here)
oh, and if you want to run it at 1700 RPM, the rotor probably has to be laminated.