Top/Main Menu Entry

I have to port an existing JIra-Plugin to forge which creates a new Entry in the main menu:
Screenshot from 2022-06-06 13-58-56

Is this possible in UI-Kit or with Custom-UI?
If not: is this planned for the future or will this never be possible with forge?

The closest thing in Forge is jira:globalPage which can have subpages in the sidebar.

You can check out the project pages example app and just change the module to a global page.

Are there any plans for a module which makes it possible to create a menu like shown in my screenshot or will this not be possible even in future?

Unfortunately we do not have plans to create a menu as shown in the screenshot shared by you.

By using a global page module your app gets a menu item in the Apps drop menu. When clicked on it the sidebar could show the other pages.