For example, let's say I would like to not use a GC memory card and instead want to use memory card emulation on all of my games using Nintendont. How can I transfer my save data from my GC card to my HD?
Try using the Homebrew Browser to find a GC memory card manager. There are a few available that will copy the saves to the Wii's SD card. Personally, I use the manager app called GCMM. You may need to convert the saves somehow, but I'm not certain. To be honest, I still use my GC memory cards...