Good Morning, Tormod!
Yes, every teacher can post to the news forum. If you restrict this forum, it only means that students can't post at all ("no discussions, no reply") or that they can't start new thread but reply to already started ones ("no disscusions, but replies allowed").
You asign teachers by first entering the course to which you want to add a teacher, then you scroll down a bit till you find in the box "Administration" on the left side the link "teachers" (it might be called something else depending on how you named that group). Now you will see a site that will allow you to add a teacher.
I hope this helps , Anya