K to add Pokemon I think a program called Unlz.gba came with it. You can't add Pokemon, but you can edit them. Ex: You can change Scizor to Arceus. Watch the youtube video "how to change pokemon sprites" or something like it. If you have trouble, respond on my profile.
A rom is a game. Like Firered for example. You can find one on google but never post a link to Roms here. You can get in a lot of trouble. you're better off hacking a GBA game. Trying to hack a D/P rom is practically impossible.