I haven't been able to convert .fbx or .abc with usdzconvert.
Have you tried importing the .abc to a SceneKit scene and exporting the usdz from there? I have had (limited) success using dae files with this method.
After looking in the usdzconvert file, I can say that .abc is not implemented yet, meaning there is no convertor for that extension in that code.
Does anyone have any idea as to when this convertor will be updated to support .abc files as well???