[TYPO3] Adding BE users to FE groups

[Robert Markula]

A nice hello to everybody!
I'm currently struggling with the following problem:

I want to add backend users to frontend groups, so that backend users 
can access certain FE (access restricted) pages. I want them to be able 
to login to these restricted FE areas using their BE password.
In times of the 'single sign-on' buzzwords I don't want them to register 
as FE users with an own username&password, when they already have a BE 
account.

Do you know of any solutions?

Kind regards,
Ro