But would a rotary converter be able to work in reverse to the "normal" sense and convert 3 phase back to single phase combining all three phases into one
by the time you have bought all the gear to make a rotary converter
it would have been cheaper to replace the altenator on the genny with a single phase
Actually you would just be using a single motor as a phase converter.
You would run it as a 3 phase and tap the single phase off of one winding, or from the mid voltage tap of the single winding.