A power may reduce an appropriate castle and then add steps to the FP (7.3.1). Appropriate meaning of the correct side (Hospitaler, Crusader, Seljuk etc.
Yes, you must reduce Hospitaler castle, to give RP to Hospitaler...
Then, may it place units in a besieged city, or a besieged castle (7.2)?
You cant place units in strongholds that is under siege. But Units that is under siege but not under blockade may receive RP.
Do these units have to go to the FP first?
You may flip unit to full str. on the map if you want.