icons_geom.blend - how to move a new icon into the export collection? 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.
I was once more beaten by the obvious here.
Have a look at the Properties editor at the right. There is the Add to Collection button in the Collections panel in the Object properties of the selected object. And here you can find the Export collection in the list then.
Thanks goes to HooglyBoogly at Blender Developer here.