Do you mean creating a render for compositing into an image of a doorway? Is this also for animation? There's no way that I'm aware of in Poser, be it features or material tricks, that allows you to "occlude" certain portions of rendered objects in this way for compositing purposes. The way it's done in other apps is with special "matte" composite materials. You would basically insert a "mock up" of whatever object is in your background image into the scene, then apply a special matte material to it, which would render invisible, but also occlude objects behind it for seamless compositing. Poser simply doesn't have this. You can, however, make objects into "shadow catchers" for compositing shadows, if you have P5/P6.

---

Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.