How can i add it to my WiiU, sir?
Like godrebourn said, savemii mod is the way to go
If you haven't used it before, you place game saves in the backups folder and in the game title save folder you create further individual folders for saves within the game title folder and you can add multiple save files if you create multiple folders, a folder called 0 would be slot 0, 1 would be slot 1 and so on.
so a Mario kart 8 save path in slot 0 would look something like this on your SD card:
wiiu\backups\000500001010ed00\0\80000001
Your Wii U may name the 80000001 folder differently, mine calls it 8000001f, create a backup of your existing save and check to see how your Wii U names it and use the same name for the folder containing the save files you wish to install.
Im told that the differing names for the save file folders is to do with the number of Wii U users that have been created on the console.
In Mario Kart you use the 80000001 folder from the save you are using but put your original 'common' folder with it, not sure why but thats what i was told to do with Mario Kart 8 but it may not be necessary as some other games only have a common folder, Yoshis Woolly World is one such game and so thats the folder you need to copy over.
Once you are ready to install the new save file, choose the wipe save data option for that game before you install it.
Hope i haven't just made it sound more confusing lol.