Import your Obj file into the list on the left. wait for it to refresh (or click the flag), and then copy the string that generated below. Once you have the string below, copy it. In glue3D, press the "page up" button to show the engine files. find the model that you want to import the Obj into, and replace the data under the "model" header with the string you copied. Make sure you're not editing the model too, or you'll overwrite the Obj with the empty loaded model. that's it!
This is experimental, It isn't guaranteed to work with all Obj files. It can handle quads and tris, But N-gons aren't supported. It supports groups, and converts them to glue3D groups too, that work with bones. however, no multiple textures. all of the quad's textures will be set to one that you choose, you can find it at the very bottom of the convert script inside. thanks to @GarnetonScratch for the amazing 3D cat model, and for @dnduality for making the model easier to import and edit for glue3D. Let me know if you'd like to see him as the default character in glue3D!