What you need is an isolation transformer (either 110v to 110v or 220v to 220v or some combination thereof) rated for the power required (if 220 to 220) or half of it.
If you don't need to balance the load between the split phases, the easiest to find and also the cheapest variant is 110 <=> 220 auto-transformer as those used to power US devices in Europe or the other way round. Use it to convert one of the phases to 220 and run UPS off it.
And, be aware that a lot of UPS devices are quite picky about their input power so it may not be the GND the thing they don't approve of. They refuse to go "online" (i.e. power the load from the input) when the frequency is not stable enough, or the harmonic content is not low enough, or the voltage is not stable enough, or whatever. Most off-grid generators fail some or all of these requirements and a transformer can in fact worsen the power quality (from the UPS view point).
Practical experience 1: all UPS devices I worked with run happily with GND disconnected or miswired to hot (many thanks to local electricians for making me check any and all receptacles in all server rooms I ever entered in).
Practical experience 2: Almost all UPS devices I ever tried to run off a generator didn't like the generator power. Exceptions: either the generator is inverter-type, the generator is in 100s kW range and powers significant deal of other (stable) load as well, or the UPS is of those expensive full-conversion types.