A token check says cameos is not a mechanic I am not aware of so OK then.
What sort of things to do you want to add and where? Most of NSMB on the DS is 3d modelled rather than 2d sprites, despite what it looks like, so you will have to make 3d models of the thing you want to add, though you might get away with assigning a texture to a square or something. I am not sure what scope there might be to add a whole new other model in the game, though there might be ways around there.
From here it is "ROM hacking - most of it/learn it" so I am not sure what you want me to cover. That said get one in there and you can get two easily enough.