No it doesn't work like that. Imagine that you're changing the code of the game. It's risky; if you stuff up, the game may not run. It also takes a long time, is difficult, voids your warranty (I believe), and prevents you from going on Xbox Live (I also think). Trying to modify your Xbox isn't a good idea unless you have first-hand experience with it.
"The only measure of a man on the internet is whether or not he has been sig'd" - adifferentcity
I don't think it is that bad. Firstly, all you're doing is modifying some info in your save file. Eg. Instead of the save file remembering your location at Skingrad, it might place you in the Test Area. The area exists normally, you just cannot get to it, it shouldn't void your XBL or warranty.
Actually, I've been hex editing this game quite a bit, recently. It's not as difficult as you may think, but if you don't know how to read hex code, you're probably screwed.
I don't think I could go into detail about it, but I'll just say it's completely possible with A USB drive, Hex Workshop, and Modio. Oh, and you need to understand hex code, which I don't think you do, with all due respect. This isn't one of those things where you can just open a cheat menu, and select what you want. it takes a lot of patience, and it probably the most technical, yet powerful way to mod your game save.
Also, @ Sinroth, this definitely does NOT void your warranty. All I have done to my xbox so far is pulling save files off of it with a thumb drive. You don't even need to open up the xbox. You may be thinking about flashing an Xbox, which has nothing to do with Hex Editing. Flashing your Xbox is for editing the actual game code, whereas hex editing is simply editing your save data that operates within the bounds of the game code.
"If you fail in epic proportions, It may just become a winning failure" - Rule 16 GT: DontEatThatPie