Would this be your first romhack?

If so, here is my advice to you: forget it for now. It's far, far beyond your skill level. You're a kid who's just about to buy his first chemistry set so he can make a nuclear reactor.

Anything is possible in romhacking with enough time, effort and cursing. It's just that it takes significantly less time, effort and cursing if you know what you're doing to begin with. Also, baby steps to start with. Not to drop another analogy on you, but you're trying to get from Los Angeles to New York City in a single step. Work on smaller, easier projects first, then work your way up. Provided the graphics are uncompressed, you could start with some basic sprite hacking, maybe try some text editing, then work your way up to the ASM-heavy stuff.

Be nice, Ryusui. Don't scare the new guy off.

DeathAngel, have you read the Getting Started section? There are some great documents there for beginners, and you'll probably get a better idea of what you're getting yourself into, and how to go about it.

The Newbie Package of REQUIRED Material

ROMHacking.net FAQ: You ask, we answer!
ROMHacking.net Getting Started Section: Newbies Go HERE!
ROMHacking.net Documents Section!
How to ask questions the smart way.
On the Essence of ROM Hacking

I've thought of this same idea before. And infact through some pro action replay you can make it look as though you are playing as Noob Saibot, Smoke, and Jade. You can even play looking as though you are Noob Saibot yet using Sub-Zero's powers.

The reason the game crashes if you just put in a PAR code to make yourself a section character is most likely some sort of lookup table of functions the game uses to handle certain things about your character. You may have noticed that atleast with these 3 unplayable characters, you can actually play as them so long as you don't hit any buttons other than I think Up, Down, and High Punch. There are a number of values involved, but if you freeze the right ones, you can make your character say Noob Saibot, however changing another value at the same time can make the game handle it as if you are Scorpian and not crash.

Anyway, if you want to attempt this you need to learn some ASM hacking and programming for the SNES CPU which is similar to the 6502. If that's over your head, you can just wait till I get back around to the idea and maybe I'll come up with something. I'm guessing you are like me and ever since playing MK2 on SNES you wished you could play as Smoke or Noob.