It is possible to insert items into the factory from the left and right side, but only 4 of each side are able to do that. Bottom and top wouldn't work. The items are displayed in the alt-overlay and you can pull them out again. After entering and leaving the factory, these inserted items dissapear.
That is possible bevor and after the "External overlay display" - research.
If the research is completed and there are already 4 items in the overlaychest inside the factory, the inserters are unable to put further items into the factory.
It is possible with the factory Mk I, not with the Mk II and Mk III.

https://drive.google.com/drive/folders/0B2GL-SKA--M_LTl6TTdHc0hhaFk?usp=sharing
Here you can finde a screenshot with the inserters on each side.