How can i make a shader that "shaders" on the whole screen? For a motion blur-effect or something? Is this possible?

Greetingz,
HeLioS

you can use the render to texture function (view.bmap) and render that on a quad in front of the camera the same size as the screen.(only if you own a6Pro..if you own a lower version you can use bmap_for_screen but it's slower)
then apply a shader to that quad..then you can do any kind of image processing you want to ..this is how other games achieve these effects.