It could be done yes.. and it would probably take a lot of work.
Someone could, in theory, convert some SP levels from JK into MP coop maps. Heck, many of the levels have been converted, but the AI, respawns, etc have not been modified for multiplayer. Some levels are more difficult because of various scripted events, etc. So it isn't simply a matter of writing one or two cogs and slapping them in there.