DS undubbing is actually pretty easy to do, you just need the right tools to get into the ROM files to play around with things. A good example of a tool like that would be dsbuff, or DSLazy. I prefer dsbuff myself.
All you do is you unpack the ROM files from both versions, grab the sound data out of the JP version, go into the NA/EU version's folder and replace the NA/EU sound data with the JP sound data. Then you repack your NA/EU version into an .nds file. It's really that simple.
The problem you run into is when the sound files for one version have different file names, which makes everything a whole lot harder. The way the sound data is included from game to game can vary a lot. The easiest is when you just have "sound_data.sdat" as the sound file.
You can get dsbuff here if you want to play around with it. I can give you more detailed instructions if you do want to actually try undubbing, or there are plenty of simple guides to be found with Google.
http://wb3000.nintendo-scene.com/dsbuff.html
I did a little bit of looking, and this seems to be an undub of Luminous Arc 2. I tried it out on my flash cart and it works fine. The game went to a black screen when I first tried it, but when I tried it again it loaded normally.
http://www.mediafire.com/?pm9cgm1fzim