I'm going to assume you're using the default configuration for DeSmuME.
After you save, you can either use the ".dsv" file that is your gamesave or you can export the raw ".sav" file (In DeSmuME, File>Export Backup Memory).
I've tested both, and it seems to work just fine either way. Just make sure you keep a backup copy, just in case something goes wrong.
You can download the newest version of the English translated Pokesav B&W from jsparrow's post:
http://gbatemp.net/t256531-pokesav-black-and-white-translation-with-upcoming-mods
All you have to do is open the .dsv/.sav file in Pokesav, edit anything, then save.
Just make sure you have the save's file name the same as the rom's file name, otherwise your game wont read that there's a gamesave.
Now I have a question about DeSmuME: Can I access the Dream World in DeSmuME? If so, how? (It requires a wifi connection, right?)