Technically Yes You can
You will need to lern a few things like Hex, get yourself a good hex editor i use Hex Edit maby checkout the Rom Hacking Section
when hacking roms you can actually use Hex and these tools to modify the game rom edit the levels change pokemon names add more change in game text etc all the stuff you want to do
then if your really fancy you can buy a flash card and flash writer and actually put your game onto a cartridge and play it on a REAL Gameboy
hope this helped you
Oh i will add that this is illigal to modyfy copyrighted material.
if you want to distribute your hacked rom to people and dont want to get law trouble you can use a IPS Patch creator to create a patch for the Pokemon Roms and then distribute your patch the reason this is legal is because you are only distributing the things youve made when the USER patches the game the illigalness passes onto them and not you.