would it be possible to add a new sprite into a super mario bros ROM. like, for example, a gigantic goomba? because i think that would be really cool!

Could you please be a bit more detailed. Btw I would think...no. You would have to be the creator to do that, I would say.

i mean like an entirely new enemy or character in super mario bros

Hmm, well I'd imagine its possible; you'd probably have to hack into the Rom somehow. The only problem is, I have no idea how you could do it.

Well if you manage to hack the only real gaint goomba you could maybey put in would be the boss in world 3! You couldn't make up your own or put one from another game, like say super mario 64. Maybey there could be away to add one that was gona be there but they took it out. Like in Super mario 64 there is a lava monster from yoshi's island that was saposed to be in Lava land but they took it out. I saw someone hack and bring it back! So I don't know maybey you could and maybey you couldn't.

There are hacks of roms around, so yes it would be possible. Would take a fair amount of knowledge of how the games work and everything though.

Yes a a brave amount of knowladge, or you may beable to find a download of a hacked one somebody already made. Hey try out my made up level. Theres no ground!

Check out some beta images from Paper Mario, (courtesy of Devil Mario):

http://nintendoextreme.co.uk/pm64.php

(Scroll down to the bottom of the page).

Those beta images look cool, mind you I never actually played the mario 64 version of paper mario. I only have the wii one

Awsome! I waaant it!