The keyword here is "yet", and it is a big "yet". It is theoretically possible to mod marth into the game. Practically speaking, the level of modding done to 64 and the knowledge of how to mod this game doesn't allow for this to happen. Additionally, even if the knowledge was there and there was someone willing to do the work, the effort to mod a character into the game is much more difficult than the other games as 64 games get compressed and there are size limitations to the rom. On top of the modding itself, I'm pretty sure the game would need to get recompiled and changes to the games engine would need to be made to support the appropriate mod. I have not ventured into modding at all but I am an experienced coder so this is my speculation of it all. Certainly anything is possible, but probable and feasible are much better words to use when inquiring about requested work to be done.