Well, TES music from III and up anyway.

I am looking for a mod that will add the scores from Morrowind and Oblivion tot he game. In other words, battle music plays at appropriate times, explorations music, etc.

Does such a thing exist? If not, how hard would it be for this to be done?

If it does, I don't think we'd be allowed to talk about it here. Straight-up distribution of the files is illegal, and otherwise copying the files from installed copies of the games would still break Bethesda's "no transfer of assets" rule.

Someone could of course distribute the "playlist" info, ie. what files go in what folders, for people to apply themselves.

You can not make a mod that uses assets from any other games in this game. Even other Bethesda Titles. Nor can you make and distribute a mod that requires resources from other games.

Now, that being said, you can make a music mod that adds whatever music you want to the game for your personal use. You just can not distribute it.

Check out this mod - http://skyrim.nexusmods.com/mods/30568//?

No need for the "OMG COPYRIGHT!!!!!!1oneeleven'1'1!"!!" because it has instructions on how to take the Oblivion music \Data folder and Morrowind music \Data folder and make it useable to the mod itself.

There is also the Personalized Music Mod, which may be a bit easier for you - http://skyrim.nexusmods.com/mods/13286/?