it is possible...but will need tons of unit change and player alliance editing... you simply use "neutral passive" "nutral hostlie" and "nutral victim/extra" and set there allys and evemys with alliance triggers... also all unit for neutral extra that need to be on the map when the game starts will have to be controled by someone else and then passed to neutral extra with a trigger...
the main problem with all this is that all computer players will look the same (unless color set triggers work on them too...i am not sure...)
victem and extra are neutrals that seem to be not normal...you cant place them on the map but you want change things to be owned by them with triggers and there allainces too so infact thay provide 2 more computer controled armys. pretty usefull if you need many computers for you map and you dont want to waste human player slots...