Can a 240V 3 phase input battery charger be somehow used with only 240V single phase?
A simple (e.g., no power factor correction) 3-phase charger or power supply generally has a 6-diode bridge as the first item (perhaps after some filtering components) on the input side to create an internal DC bus. You'll want to verify this before proceeding.

If you connect a single-phase source (of the appropriate phase-to-phase voltage) to two of the input terminals, you will be using just 4 of the diodes. This should work just fine, as long as you keep the load to 2/3 of the 3-phase rating, so as not to exceed the capabilities of the diodes.

If the charger is a modern one with power factor correction, the input circuit will have 6 MOSFETs (or maybe IGBTs) instead of the diodes. In this case, you'll have to ask the manufacturer about single-phase operation. Depending on how they sense the currents and voltages for the PFC function, it may or may not be possible.