Mixed retained mode and immediate mode rendering?

Does JME provide the ability to mix scene graph rendering with immediate mode LWJGL rendering?

Yeah, you need to override simpleRender in Simple***Game if you're using those classes, implement render in Pass and add it if you're using SimplePassGame, or override render in GameState if you're using StandardGame. Then you can use Renderer.draw to queue the scene graph for rendering, and call Renderer.renderQueue to send the commands to GL.

jME tracks the OpenGL state internally, so after you've made the immediate mode calls, just make sure to tell jME these changes have been made by either updating the appropriate state record or, the easier way, tell jME to invalidate its knowledge of the OpenGL state machine.