Custom Preset Avatar models

The Meta Avatars SDK includes 32 preset avatars that can be used OOTB. I extracted the PresetAvatars_Quest.zip file from the SDK and saw a bunch of .glk files. However, it looks like the model files are in a customised format after I probed with the file utility: "Zstandard compressed data (v0.8+), Dictionary ID: None". 

GLK files exported from Ready Player Me don't load at all. How can I create my own preset avatar models?