Yes, Bell Scooter can be used in the XMAX 250.
XMAX uses a standard CVT layout where the clutch is not bathed in engine oil, so the additive will not cause clutch slip.
TMAX, however, uses a wet multi-plate clutch located between the engine and CVT, and additives can change its friction characteristics.
This is why TMAX is not compatible, while XMAX is safe.
Recommended dosage is 5–10% of the engine oil volume.
Simply add and ride normally; the lubricating film forms gradually through operation.