Start pump with grid then switch to generator to avoid starting current

This can be done. Transfer the motor load to DG Set through fast transfer & with the help of sync-check relay. See the capability curve of DG set for kW/kVAR loading. Never try manual transfer. If pump has to be started frequently don't try this method. Pl try variable frequency drive for biggest motor starting on existing DG Set.

But it is not a good idea to transfer the motor load from a main supply to a generator supply unless the two supplies are synchronized. Even then a transfer would take a finite time during which the motor would slow down and would effectively restart on the generator supply which could cause it to trip.

I would suggest that you look in to different ways of starting the pump motors including
Star/Delta or some form of reduced voltage starter (e.g soft starter). Alternatively you could look at starting the motors on no-load, this doesn't reduce the starting current but it does reduce the time the motor takes to reach full speed.