I've never heard about injecting Wii saves, but Wii System Transfer transfers all save files stored on the Wii System Memory to an SD Card, which is then inserted into the Wii U to be transferred to the vWii System Memory.
It's basically like DSi to 3DS Full System Transfer except the saves are not deleted, it involves two home consoles instead of two handhelds, you need to manually remove the SD Card from the source system and insert it onto the target system, and the transfer happens on a bigger scale.