I think megaman 9/10 had some issues and wasn't working with simple emuNAND.
so, neek is the solution.
but I don't know about DLCs, there's no reason they don't work. You can install them anytime you want, with or without a savegame already existing.
the DLC goes into a different folder, and the game look into that folder to load found dlcs. if they are from the same region than the game, there shouldn't be any reason it's not detected or working.
you can try to install the DLC using ShowMiiNAND (update for vWii), maybe the loader is not properly extracting them?
without proper debugging, I can't tell.