I remember seeing a while ago a JMonkey library on github dealing with creating custom geometry,
it was a wrapper around the Geometry, Mesh etc. stuff.
It had class called Vertex.
Now I thought I had created a bookmark but apparently I didn’t,
and I can’t the library anymore.
I’m not sure what you’re looking to do but rolling your own geometries isn’t particularly hard from scratch. The tutorial from the Wiki is pretty straight forward. It took a little bit of fiddling but I was able to write code to generate all sorts of whacky procedural shapes (asteroids, an extremely divided icosahedron [icosphere] etc.).
Otherwise if you’re just looking to make some static shapes that aren’t procedurally generated I’d consider possibly just cracking out the models from Blender.