Is there a way to use a 3d fbx object as a light source?
Models aren't lights. Lights are lights.
You can always parent a light to a model (inside the model if Cast Shadows is off), but, if you're using a model with FBX animation (like walking/dancing etc) you'll have to animate the light to follow the animation.
@Triem23 you can actually fake lights by a glow texture but I don't know if you can transfer that data to fbx or not?
@Andersen01498 true, but I'm assuming OP wants to illuminate other objects in the scene.