export your video as a rpf sequence and chooose z-depth as render pass.
or add it as a render element so that you have seperate pass of zdepth.
then you have to use aftereffects (or a similar program) to use the zdepth to create depth of field.
its pretty simple to do it with after effects you choose lens blur and use the zdepth layer as referance.
if you have more questions
andrew kramer has a good tutorial about it
http://www.videocopilot.net/tutorials.html?id=101if you want do it via max you could use final dof
or if you want it totally manually with max camera then you better have a good renderfarm