Java doesn't use direct converts from commercial 3d software suites, it usually uses actual programming for each model, placing each model through a xyz system of matrices.
This could be auto-generated with proprietary software made by jagex, and if that's how they are making them, then odds are you're screwed.
3d models are just a set system of verticies positioned through matrices telling where each point is from the orgin of the 3 spectrum from with the object exist.
So if you use the formula matrix
[2]
|2|
[2]
It's saying the verticese is 2 up from orgin, 2 to the right from orgin, and 2 further back from orgin.
The point of explaining that is, each format contains different instructions on how to render these instructions, SO, if Jagex did indeed make a proprietary suite that creates these instructions (they probably did, java isn't big with 3d), you would have to backwards engineer the instructions in the model's file, then find a way to convert those instructions into another format.