There are -to my knowledge- no way to have more than 7 playable tribes in a single game. But you have some options.
1) Let the barbarians simulate an extra tribe, or a number of minor powers in the game. -set the barbarian activity to "raging hordes" and give them cities. You can do this by creating an empty city, and create barb units next to it. This is easiest if you do this in the early stages. Otherwise other players will take turns moving units etc. which is undesirable in most cases. You can also create barbs with events etc.
2) make one Civ a "neutrals" tribe that counts for a number of minor powers in the scen. This is best used, if you want several additional "civilized" -less aggressive- minor powers.
3) make several scenarios with differing tribes, depending on your setup and range of the scenario.
Personally I prefer no.1 option. Barbarians are difficult to control, but you know one thing. They will keep coming, attack and attack again and again. Actually they can be a quite awesome opponent with a few great cities that keep spawning strong units. Notice they will usually produce the same unit that conquered them. -Use Carl Fritz' utils "CivTweak" and "Civcity" for altering some game parameters, if you need changing some of this.
I know of no ways to use more extra units than the slots available in the units.gif and rules.txt. -there are 3 extra slots in the classic version, and, I think, about 10 in the MGE version, that can be used. -But that's it.
Best of luck,
Morten