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Well, talking about fire red there is a patch under development (know as artificial red) but it is still not available in public.
To do so on ur own you have to expand the pokedex by gen 3 hacking suite (aka lord of the tools) and then insert sprites along with their information, evolution and attacks etc.
Sorry, but I think there is no "easy way" as of now and pls correct me if I am wrong.
I also think that in ROM hacking shortcuts lead to disasters.
It is definitely possible, and most of it's not too difficult with today's knowledge, but it's a HUGE amount of work and there are still some technical challenges to work out.
Several people have attempted this over the years, but no one has finished (or at least no one has released a finished version). Epsilon is working on a FireRed one right now (Artificial Red) that looks extremely promising - follow his progress here.
There is also a released patch for Emerald that contains the first 650 Pokémon (Gen I - V) - here's the thread. I don't hack Emerald so I can't vouch for it personally, but from what I've heard it's pretty stable.
Honestly, if you want this in your hack, your best bet is to wait for Epsilon's Artificial Red patch, or make do with Emerald 650.