Per shortbus, most large industrial contactors, depending on make and model, offer a coil kit. Should this be the case you simply remove and replace the coil with a different voltage coil. Again, this depends on the make and model of the contactor. When that is not possible and replacing an entire expensive contactor assembly is not possible a trick I have used would be to get a 240 / 120 volt control transformer and use it backwards applying 120 VAC to the secondary and use the primary 240 VAC side to drive the contactor coil. That works well as long as the VA rating of the transformer can deliver the needed coil current.
Ron