When you try to import material downloaded from Quixel, 3DCoat it is expecting a zip file, however the new downloaded Quixel material is not a zip file.
Now inside 3DCoat (from version 2023.40+), you can add the additional Quixel folder by following these steps:
1. Open 3DCoat.
2. Go to the Edit > Preferences.
3. Navigate to `Input/Output` tab.
4. Here, you should see an option for `Additional Quixel folder’

5. Set the path to where you’re Quixel folder materials are and hit apply and close.
If any map is named with a space in the name, the texture is assigned as new Smart Material.
Restart 3DCoat and you will be welcomed with the following:


Add existing materials folder
Also can use this other option for future needs.

Add the Smart materials from an existing folder. A new Materials folder with the same name will be created. Select at least one file to automatically add all the files from the folder to the list.

This is the complete list of aliases (at the end of the filename) used to recognize the texture usage:
Bump:
_DEPTH, _BUMP, _DEEP, _DISPL, _DISP, _DISPLACEMENT, _D
Normal map:
_NMAP, _NM, _NORMAL, _NORMALMAP, _NORM, _NORMALS, _N
Color:
_COLOR, _CLR, _DIFFUSE, _ALBEDO, _CL, _RGB, _DIF, _C
Gloss:
_SPEC, _GLOSS, _GLOSNESS, _GLOSINESS, _GLOSSNESS, _GLOSSINESS
Roughness:
_ROUGH, _ROUGHNESS, _R
Clip mask:
_MASK, _MSK, _MS, _CLIP, _ALPHA
Metalness:
_MET, _METAL, _METALL, _METALLICITY, _METALICITY, _METALNESS, _METALLNESS, _MT, _ME, _METL
Ignored:
_PREVIEW, _PREV, _TMB, _THUMBNAIL, _PRV, _AO, _OCCLUSION
The any of . – ~ may be used instead of the _ sign.