If you want this to work so that third party apps see your video as a standard webcam stream (and is sounds very much like you do) then the only way to do it is to write a virtual webcam driver. You will then then be able to "play" your video content as your webcam's streaming output. This will involve writing code to decode the video content - probably using DirectShow - and then copying the raw video stream to the webcam's output stream. You'll need to either write a custom DirectShow renderer filter or use the sample grabber to access the raw, decoded frames.
It's certainly an achievable goal, but not particularly easy given that both DirectShow and driver development have fairly steep learning curves. There are a variety of different driver models you could use for this, depending on exactly what your aims are. The easiest thing would be to create a Video for Windows (VfW) virtual camera driver. The huge benefit of this approach is that the driver will be entirely user mode code - much easier to debug and to write in general.