Indeed. Many people who only play hacks don't understand why something can't be done. While I have done minimal bits so far (map editing, namely), I still realize that not everything is possible. The limitations present in the original game itself are the constraint at play here.
Trust me, if adding every single move introduced in the 4th gen. was possible, plus adding all 4th gen. Pokes up to Arceus (493), with no removal of the existing movesets and Pokémon, there would already be a ROMhack base available utilizing this for hackers to use at will. Same with adding 5th gen.
Cyclone