In your backpack, fill it up with useables (Food, Water, Weps, Ammo Boxes, Etc). Place these items on #'s 1-9 on the action bar. Then, proceed to hop servers (or find more backpacks in the same server) and put your backpack inside of the new backpack. With this new backpack on you, you can still use the items inside your old backpack cause its considered "on your player".
Proceed to take this backpack and put it inside another backpack over and over again until you can no longer open doors on a new server. This usually happens at a threshold of 10 or so backpacks. Unsure exactly as to when the threshold is met.