As far as i know there no way to do this, property editor does not support anything like this, if i think about it it would be little iffy to implement this, as categories are not strict entities can be mixed and what about search and filtering situations?
But there way to implement this yourself, you wouldneed to create a struct like FSeperator with single uint8 (maybe there wya to create empty so we only take extra bit of memory and without UPROPERTY which would create larger memory footprint, and then… create custom UI that will create separator 
here info how to make property editor UI for your structure:
This requires some Slate knowlage, but simple SBorder widget should do the job