Basically Danny Varod seems to know as he posted it as an answer to this question:
I think, theoretically this might work, but I have no idea how to actually do it. Since I'm also not allowed to post a comment under his comment nor am I allwoed to ask on meta about how to contact another user, I ask this as a normal question here:
How do I render my own DirectX Stuff to a full screen WPF's DirectX surface?
For starters, I have no idea how to get the DirectX surface from a WPF window. If I had it, what do I have to take care of that the WPF rendering doesn't screw up my own rending or vice-versa?