You can easily write one yourself, the wiki explains how to write j3o files. The problem is it won’t work the way you imagine. The paths for images and other resources have to be set when the j3o is created, its like a compiled class and part of a project, not an exchange file format. j3o’s can even contain references to classes that exist only in the project they were created in.
No, its not bytecode but serialized java objects. Its best to do the conversion with java, as said a simple 6-line application would easily be possible. But as said, it will create a j3o file with absolute image paths. Say you create 10 models, each has a folder called “texture” next to it and inside is a “modelatlas.png” file. Then all models have an image path like “texture/modelatlas.png”. Later in the project you have to copy all files to (assets location)/texture/modelatlas.png" so the j3o will find it. From the second model on you have a problem. Same if you register each model folder with the assetmanager. As the names are all the same (the name starts at the root you register and again has to be the same as when you created the j3o so it can be found) you will always load the same image for all models.
The SDK handles all this for you with the model import tool and if you have many models to convert you can copy them into the assets folder and select multiple files to be converted.