AGK is very flexible, you will have to do the legwork yourself though.
You can run an entire custom render chain if you want to with HDR, PBR, high quality post effects and whatnot. There are some caveats, like no floating point rendertexture support, but that can be worked around.
For post effects you can render your 3D world into a rendertexture and then use that to create the final image with a custom shader on a fullscreen quad. AppGameKit provides fullscreen quad objects with CreateObjectQuad().
You can also take a look at Janbo's shader pack, I think he has quite a few fullscreen effects already covered.