The lighting.j3md only has specular, normal and diffuse. Is there any way to add occlusion and displacement as well?
You should bake occlusion in the diffuse map.
By displacement you mean parallax map?
If it’s displacement using some kind of tessellation in a geometry shader it’s not supported.
I couldn’t bake AO into my diffuse map since I was tiling it, but I found it very easy to add an extra map into the lighting shader for this, don’t be afraid to get in there and fiddle with the shaders (just be wary of updates overriding your changes, would be great you could extend a shader) 
i found using .j3m files without baking 2 maps into one makes the texture appear more realistic in jmonkey. the textures actually look more colorful and bumpy, compared to baking 2 maps into one, it looks really silver/grey.