You can continue to expand your inventory in this manner, by activating rage mode and swapping out baton for additional items as your inventory swells. That will add additional weapons to your inventory, so that you are now carrying more than the usual maximum of 18 weapons.
If your inventory is full at that point and another weapon pickup is nearby, you can quickly select that other weapon to replace the baton that John is carrying. John will pull out staff, which is separate from his standard inventory. When you are playing as John Morgan, enter rage mode.
