You need to rewrite engine’s shaders and shadow renderers.
Now: lights are drawn everywhere in the light’s radius, then, for every light a shadows are generated and drawn by darkening previously lightened pixels.
Should be: lights are drawn only on surfaces that are not covered by shadow. For multiple lights use additive blending.
It is possible, but it is a hard piece of work for someone, who does not know the engine. There is an topic about my first approach with shadows, you’ll find there a link to a project (not mine) which should be a good starting point for you. It is possible to have nice shadows in JME, the proof is here 