Is there a way to have a cooperative game of Human+HumanxCPU+CPU?

Yes, you make two human teams and two CPU teams. then you make the two human teams the same color and the CPU the same color. I do it all the time.

i wonder if he means on-line. you can't add cpu to on-line matches, can you?

quaker is right. I wonder if it's possible to add CPU players to on-line matches

Nope... No CPU in on-line maches...