So i'm making myself a private resourcepack for my city, and I want mod support for quark, so is there any way I could get mod support to add new textures to quark blocks?
Sure! It's pretty easy, and in fact the same process you'd go through to make a regular Minecraft pack.
The only difference is that instead of looking at the Minecraft files in /versions/, you can instead open the mod itself with an archive manager like 7-zip or WinRAR to see the contents.
The /assets/ folder is the same one that's already in your pack. After that you'll just need to look at what sub-folders the mod has and mirror those. So instead of "/assets/minecraft/textures/..." it'd probably be something like "/assets/quark/textures/..." Obviously you'll need to check the mod itself to see what it names its files and folders. I'm just guessing for the sake of example.