Wound Rotor Induction Motor Generator Conversion
For all induction motors, (you would have to short circuit the rotor's slip rings):
Normaly this is done by adding capacitors at the output. The rotor has to have the remanence of magnetic field, if it doesn't have then you have to magnetize it by short current pulse. This is done with 12V battery, you connect the battery on two output leads by very short time, approx 1s (in your case you choose the slip ring instead).
When you spin the motor, the remanence induces voltage in windings, but in induction motor the excitation current is 90 degrees of, so when installing capacitors, the excitation current begins to flow 90 degrees of phase and the generator voltage builds up.
Such generator is not suitable to power a starting induction motor with close power rating to the generator's rating, becuase it will demagnetize the remanent magnetization of the rotor and you'll have to magnetize it again.