Well, it copies it in a different format so you would have to use Savegame Manager GX on BOTH the Wii AND the Wii U to extract/install the save. Were you doing it this way and it failed?
Also, it can't install a save for a game unless that game has already created a save on the system so you'd have to at least run the game once on the Wii U, create a new save file, exit out and then use Savegame Manager GX to REPLACE that existing save with the one from your Wii.
As far as using Nintendo's built in tool, I think there's also some PriiLoader hack that will remove the copy protection on SOME saves but I know from experience that it doesn't work for all saves. Even if it works, I don't know if it will let you copy it BACK from the SD if it's protected.
If all else fails, there IS the Wii U transfer tool. You'd just have to be sure to backup and/or temporarily removed stuff you have right now and didn't want to be moved/overwritten