Add new external cran repo to the RStudio Package Manager

I have installed the package manger for testing.
But I need more than the default cran repo.
I also need the packages of It is an normal cran style repo.
But I can't find out how to add this.
I only see that I can add single tar balls and sources from git. But this must be done for every single package.
So adding the rforge packages by hand and package by package is not an option.

How can I add the second repo which will provide packages of rforge?

Thanks for any ideas.