As the Permission and Usergroup System of K2 only provides for the Controll of Frontend Submissions and Editing, is there any way to integrate an simple Permission system to the Backend of the K2 Component?

I need to grant my users Access to the Backend and separate them in Author - Editor - Publisher. This task seems to be not supported by K2. So i was thinking if it is possible to do this via the native Joomla Access Control System. I worked on it for some days now and browsed through the web several hours a day. I couldn't figure out a way.

Does anyone have a solution for this or a recommendation how to solve my problem?

If not is there any sophisticated Extension or Tutorial on how to edit the "Edit Form" of K2 for the Frontend? Would be great if you could share your individual solutions to this problem.

You mean that you need to give them specific access to add/edit items in categories and so, like you have in the frontend?