actually you can. The person who made desmume has made a program so you can do wifi battles. you must download the program.
Here is a video how to get it: http://www.youtube.com/watch?v=JpbTaMPDO8g
there are many videos, with different versions of desmume. If this doesnt work try searching up 'how to get wifi on desmume' on google and find other methods.
hope I helped! :)