I want to use my Canon 5D mkII as a video camera on a recorder with only an composite input. Is this possible with Raspberry Phi in the way that Raspeberry Pi uses HDMI-in and delivers composite-out? If so, what do I need for it? At the moment I am using Raspeberry Pi2 as a mediacenter and I am very happy with it. Unfortunately I have none to little experience in programming so... Pls who can help?
That is not possible with Pi, since it has HDMI output instead of HDMI input which you would need for your idea.
However, according to this specification page, it seams that Canon 5D mkII already supports composite video output, so you should better check you Canon 5D mkII cables and manuals.
It is possible but it will cost you $100s. Google "Raspberry Pi HDMI capture"
Looks like your best bet would be a new Kickstarter not as a USB device but a special board for the raspberry Pi.
If I understand you correcly you want to use the Pi to either record the video from the camera, or possibly use it to stream liveview to another screen or something?
You can use a USB TV Tuner. There is a list of compatible ones here.