LOL, there's that easy solution thingy again... I got to remember COPY PASTE!!!

Well, here it is (since their server keeps failing, I won't leave a link) ...

Instructions to those that know at least something about modding:

1.) Extract the cliff, lat, etc... piece that you will use as the template for your altered piece. Make sure that you extracted it from the appropriate .mix file, so that the file extension will be the correct one for the theater you will add it to (.des, .ubn, .sno, etc...)

2.) Open the file using Olaf's .tmp editor. It will have to be opened using "All Files", because for some strange reason it doesn't register as actually BEING a .des, .sno, .tem, etc... file.

3.) Edit the piece, remember that not all colors will be supported do to the various theater pallets. Use copy/paste to transfer the images back and forth between some sort of .pcx editing program (I'm using Photoshop 4.0 LE, Paint Shop Pro also works good - especially since you won't have to keep purging the clipboard with that! lol). Copying and pasting the "Extra data" pieces is done with the drop-down boxes that come up in the edit menu.

4.) Save the file and move it to where your going to be compiling all this (I created a folder called "dumping ground for mod" to make organizing things easier).

5.) Change the file's name by adding a letter suffix to it. For example, if you edited "cliff05.des" from the Desert theater, change the name to "cliff05a.des".

6.) Get the marble madness version of the piece you changed (not necessary in all cases, not with LAT since all LAT pieces use the same mm tile) and save it to the folder your storing all this in. Change it's name in a similar fashion to the method you used for the cliff piece itself. For example "mclif05" would become "mcliff05a". You can edit the mm version if you want, but it really isn't necessary if you know what I mean!

7.) Move this data into the appropriate theater "iso" .mix - the same place you got the original piece from (using that mix editor). The theater mixes ARE NOT the "md" versions of the theater's mix, so if you're doing a piece for the Desert theater - open the "isodes.mix" not "isodesmd.mix".

8.) Compact and save the iso mix, then move it into the ra2md.mix. Compact and save again.

Now your piece is in there. When using FA2 YR you'll notice the piece itself doesn't show in the actual choices. Select the piece you edited. Notice how the part will occasionally "switch" back and forth between it's normal version an your version. Much the way the LAT tiles occasionally change graphic on there own. If you want your piece, "jiggle" the selection til all of it's parts are your edited version. Use the same method if your want the original piece instead.
In case you're wondering. No, there is no easier way, but this actually gives you the opportunity to add something of this scale to the game! And that's all that matters.
As for maps already done in the original theater, no this won't change them. FA2 YR saves the tile specific data exactly as it appears on it's screen. So Tour of Egypt will not have bits of my edited version of the Desert theater popping up around it.

If you want to increase your chances of the tiles randomly switching to your edited version, simply resave you .tmp as yet another letter. Example, "cliff05a" and "cliff05b" with the same image for each, and the ever important mm version as well. This increases the file size (making downloading of your mod a pain to anybody) but it makes it easier on you, the map editor/modder.
If you want the original version of the theater, simply disable the mod and do it normal, like you couldn't figure that out.
I also recommend making your maps in the .map format, and saving them to a copied version of the multimd.mix (found on the yr disc). Include that with your mod, cuz it's easier just to disable the one mix file than it is to have your mod keep moving a bunch of ".yrm"s back and forth. Remeber, though, you must manually change these with notepad to say that they are "oficial" and they are "multiplayer", else they won't work with yr. People using your mod can then all have the same group of "oficial" maps for your mod, using your new theater of course!

Now I really know why nobody's done this before!!!
Whao! that's a ton of work, but I NEED this, RIGHT?!?!

- copy pasted from the entry at cncgames forum... COPY PASTE!!!