I haven't messed with the Wii U but I thought the process was similar to the Wii. The games likely need to be in the WBFS format and you would need an app that reads it (for Wii it's USB Loader GX or Wii Flow, not sure if either are available for Wii U). Do you currently have a loader for the Wii U games? A lot of times there is a tab or filter to select to show only Wii games. If that's the case, you'll need to transfer the Wii games over to the hdd with something like WiiBackupManager (again, this is for the Wii, not sure if it will work for the Wii U).
Hopefully that gives you a starting point. I wish I had a Wii U to figure it out for you.