When you find crossbow bolts you find them in a bulk, which is equivalent to a pack. These can be ejected, and you will get 1-5 bolts.
When ejected, this stack of bolts can be loaded back into the crossbow, which shows 1 bolt. If the bolt is dragged out into inventory, it's a bulk again
Now if you press "f" or scroll you can select reload with: the bulk, like if it was a quier.
How to convert Stack into bulk:
Eject bulk of crossbow bolts.
Load them into the crossbow.
Drag them out agian. They are a bulk again.
How to reload it:
Load a quiver or some other bolts into the crossbow.
Have a bulk of bolts into your inventory while crossbow is already loaded
Scroll or press "f" you can now reload with the bulk, which acts like a quiver/snap/speedloader. And only takes up 1 inventory space.