To export from Dragon Bones, export from Dragon Bones to the latest Spine format. This actually produces a Spine JSON file that can be read perfectly by the engine.
Some example models are available in the DragonBones Demos repository. You can export them to Spine JSON and test now!
Note that Castle Game Engine uses our own, open-source implementation ("runtime") of the Spine JSON format. So you can use this with Dragon Bones for free.