I'm trying to import USDZs for ARKit prototype. I followed the steps in this post, but USDZ import is not supported with this process. I'm aware that pixyz can achieve this, but I'm wondering:
Is there a free method to convert a USDZ to a Unity-eligible format?
You can't import USDZ but you can import USD. If you have an USDZ, fire up Xcode and export it to USD. Beside USD, Xcode can export USDZ to several other formats.
As you said, only USD Unity SDK was released as a Preview Package in 2019 for free. That's all.
You can unzip the USDZ file using something like 7-zip and then import the resulting USD file with the package.