Multiple opponent is a regular part of most Aikido training, and also part of Daito Ryu from what I have seen. I was taught to refer to it as taninzugake.
Below is a demonstration from Moriteru Ueshiba, grandson of the founder of Aikido:
http://www.youtube.com/watch?v=hzp8EKI4jN0The idea in practice in Aikido is awareness of movement and space and to keep "centred" in all of it. It isn't suppose to represent a realistic attack situation (if such a thing could even ever be truly created). It is the same as Tanto Randori... it is designed as a method to learn Aikido principles, not to learn how to defend againist a knife attack!!
On two occasions my Aikido teacher decided to let us go "freestlye" in taninzugake. Basically there were no strikes allowed, but stand up grappling (throws, takedowns, sweeps etc...) was allowed. No one in the class, even the Dan grades, were able to keep away from a group of four opponents (standard grouping for the excercise) for very long.
It was real eye opener. A guy who was a 2nd Dan and trained MMA for 4 years found it just as tough as some of the greener students. The teacher of the class said he did it to show us how incredibly difficult it was just to survive against multiple opponents, let alone try and "beat" them.
I found it had a lot of value. It helped you avoid movement from several different sources, helped you develop better awareness of space and helped you to stay calm and balanced in the midst of the chaos.
It goes without saying that trying to use Aikido to "beat" multiple opponents never worked. The people who focused on avoidance usually lasted longest, whereas those who tried to engage their opponents got overwhelmed pretty quickly.
Trying to fight multiple opponents didn't get you anywhere in my experience. If you are to train it, I would try and concentrate on avoiding their attacks.
Another strategy I never tried out, but was suggested on another forum, was to use the clinch to tie a single opponent up, and use them the shield yourself from attacks. Nice idea, don't know how it would play out in training though, and I don't know enough about CIMA to know if there even is a clinch-type movement in it.