I don't use Facebook, but just from intuition it would be nearly impossible to add yourself back as an admin without the help of Facebook itself. If this was your website, you could directly edit the code, giving yourself administrator privileges. But since you can't access the code of Facebook, you would need some sort of exploit. And that would most likely take months to find. It's a better idea to just contact Facebook and request that they add you back as an admin within the group. As long as you're polite, provide them proof that you were an admin, and how you accidentally removed yourself as an admin, you should be added back.