in Blender duplicate the Object to another layer - rename it to YOURMODELNAME_L0 - YOURMODELNAME_L5 (so you have 6 duplicates)
Create a hitcheck (its just a box roughly the shape of your object) call it: YOURMODEL_hitcheck
select your model + all LODs + hitcheck model (REMEMBER TO SET ALL LAYERS THAT CONTAIN THESE OBJECTS TO VISIBLE)
then export as .fbx (check selcted only in export panel)
Levels of detail
For engine performance reasons every scenery item must have 6 level of detail (LOD) models.
The Level of Detail or LOD is a process of swapping models to 'less detailed' models that look the same over distance, as the camera moves away from the model the simpler models replace the more complex model. This allows for detailed models to be placed in game while maintaining good performance.
There is a total of 6 levels of detail that are always required as part of the model or it will fail submission.
LOD_0 - Highest detail model 100% triangle-count
LOD_1 - 60%-80% triangle-count of the highest LOD
LOD_2 - 40%-60% triangle-count of the highest LOD
LOD_3 - 20%-40% triangle-count of the highest LOD
LOD_4 - 10%-20% triangle-count of the highest LOD
LOD_5 - 0%-10% triangle-count of the highest LOD
TLDR: LOD level 5 is when the player is really zoomed out, LOD level 0 is when you are right on top/or-near the object. You want to optimize your LOD levels, otherwise you introduce very notable FPS lag when you layer your object in with everything else that a person has placed in the game.