The Blender toolshelf icons are defined in a icon_geom.blend file. The file can be found here: https://developer.blender.org/diffusion/BL/browse/trunk/lib/resources/
This blend file has a collection called Export. And a new icon needs to be in this collection. But it is not described how to do so. Just that it is required. There seems to be no UI way, the collection just shows in the Blender file and Orphan mode in the outliner. It is not in the move to collection dialog and not in the Scene or View Layer mode. Drag n Drop fails, paste fails. I have tested everything i could think of. No avail. I have never seen such a hidden collection before. It is not documented.
So how do i add a new mesh object to the Export collection? Any help is appreciated.