Is there any way to use the texture packs with existing Citra installations? I use an emulator front end (Retrobat) that uses Citra-qt.

Yes that's fully possible! However, that Citra version has to support Custom Textures (usually the option is found in the Citra graphics settings > "Load Custom Textures"). As long as that works, simply copying the "user" folder from the texture pack directory over to the Citra-qt's directory should