Short answer: Choose an existing tribe.
Long answer: No, you can't just make one up off the top of your head. However, if you really feel your character concept doesn't fit with any of the existing tribes you have a few options:
1. Your character left their tribe and doesn't have one. This may have in-game consequences with other barbarians, but is certainly within the rules.
2. If want to make up a new tribe it must be approved by the plot team. There's a great deal of thought and work and balance put into each of the existing ones, so adding one is much more than just making up a name, and altering a race packet is not something we do lightly.
If you want to talk about option 2, shoot me an email (allianceplot@gmail.com). We'll see what we can work out.
-Michelle
HQ Head of Plot