Just tried with Mighty switch force, on USB from sysNAND: worked successfully!
To make it clear for anyone confused (supposed you're on the wiiu menu and you have the CFW Booter homebrew on your SD Card):
1) put fw.img (any should do it, but be careful where you find it) to the root of the sd card
2) Launch Internet Browser then Homebrew Launcher (on loadiine.ovh)
3) Load CFW Booter, the screen will become red with "Welcome to redNAND" then "Initializing" written on it, don't worry, it's gonna launch that fw.img file
3a) Wait like 10 seconds, the wiiu will reboot to the menu.
4) Launch Internet Browser and the Homebrew Launcher again
5) Now Load "WupInstaller Mod y"
6) Press X to install your Eshop game/DLC on your USB or external HDD
6a) If it worked, you should see the install is in progress
7) Once done successfully, press home, you're gonna be redirected to Mii Maker: press Home then WiiU Menu
8) ????????
9) Profit !!
I know my english isn't that good for tutorials, but that's how i proceeded accorded to what Kohmei said, just wanted to make it easier and clearer for you all, i hope it is now~
PS: The game/dlc will probably no longer work once you shutdown the console, if so:
Repeat the steps 2 and 3 then lunch your game/dlc from wiiu menu. Simply easy.
PPS: Once you did steps above, as long you keep your wiiu on, you can just repeat steps 4 to 7 for each content you want to install separately.