How can I supplement a portable generator during peak load.
Member mkeith makes a good point - you can use another induction motor to supplement starting current requirements.

However, you actually want to use a fairly large motor, or a smaller motor with a heavy flywheel attached to the shaft.  It is the rotational energy contained in the spinning rotor and flywheel that provides the extra surge current capability.

Fairly large motor, to me, is anywhere from 1 to 5 HP.

Also note that this extra motor MUST be an induction motor, not a brushed motor.

A really quick way to try this is to use a decently-sized bench grinder with dual 8" wheels.  The grinding wheels act like flywheels and pretty much every bench grinder that I've seen since I was a kid uses induction motors.

It's certainly worth a try.