If I had the Wii version I woukd look into this.
Simply making a 3DS Xenoblade save editor would make this project a lot more feasible. Documentation is the main thing getting in the way. Both save formats need to be fully documented before you can write a converter.
The first thing I would try is copy and paste, if it is the same save code (minus the apis) it *should* just work.
Yea. I'm not sure how much code was reused, nor am I familiar with how Wii/3DS saves are stored. But I'm willing to bet that the general structure is probably pretty similar give or take a bit. Make two identical save files, then compare the data. A direct copy+paste might be possible. Might need to change a header or something though.
Have a Wii save?
yes if you want
Nope. I'm not even interested in the game. I'm just here because the idea of converting a save file is a bit intriguing. I know Gamefaqs has save files for stuff: http://www.gamefaqs.com/wii/960564-xenoblade-chronicles/saves. Again, I'm not that familiar with 3DS/Wii saves, so I'm not sure if there's some standard you should be looking at.