How to insert/replace coding in a Super Mario Bros. ROM?

[ChaoticFox]

It's exactly as the title says. I wanna rewrite the coding for one of the bricks to make it hurt Mario, but I have no idea where to start. I already know enough ASM to pull off something like that, since I inserted objects all the time in my SMW hacking endeavors, but that game had assemblers that would insert the coding for you, where as this game... doesn't.

Was I clear in what I wanted to ask? I think I might've rambled, but I want to make sure I don't get a "lern azm" reply 

[Trax]

You can't because the ROM's banks have no unused space left. If you can find a way to expand the ROM and convert some code to bank switching, then you could add all the extra stuff you want...