Render Depthvalue-texture to framebuffer

Hello, I need to render a depth-texture to a framebuffer-object. Each texel should take both the maximum and the minimum depth-value of a certain search area, so I need a shader which does the depth-lookup and finds the maximum and minimum-values.. Has anybody yet implemented a depthbuffer-renderprocess using Framebuffer objects and a shader? Is it possible to use shaders when rendering to framebuffers and how do I setup a framebuffer which takes the result of my rendering-process? Any help on this is appreciated, Icebraker