The problem is, that the two major teams that released cooperative mods for Doom3, have still not released their codebase even though there hasn't been updates made to their projects within the last couple of years.
All efforts in convincing them to do so have failed so far and so I started a new Open Source Doom3 Coop coding thread which gives ppl, that want to start a new open source approach, all the tools required.Link
to the "The Doom3 Open Source Coop Coding Thread"
You can go there and get working with a codebase that provides basic clientside coop support and integrate/enhance it with your own source.
It never ceases to amaze me how consequently the majority of the d3w community simply ignores Doom3's multiplayer/cooperative capabilities and how little open source community content is being created in that direction, not utilizing this truly awesome aspect of the game.