*puts on his archaeologist hat*
okay from a recent lesson on this very subject.
Nomads often supported cities due to the fact they had most of the livestock (at least in the Sumerian era ect) they're often hated/feared by people (look at modern Iran) have very different customs ect, BUT they are very useful to the cities due to the fact they bring in resources from distant areas... so they're most useful purpose could be to bring in things that your forts site exploitation area (the bits you can physically get goods from) doesn't have, so if you have no volcano they bring obsidian ect ect.
Nomads do often trade for what they need for what a city needs, so if a city needed say flint (which wasn't uncommon in some parts of the world) the Nomads would go to one of their areas, collect that by trading for what that area needed, and then return to the other city, sell them that good in exchange for what they might need but are unable to produce for themselves (more advanced weapons for example).
Its rare that a nomadic people ever grow large enough to attempt a full scale war and conquering of a urban based nation, (though it does happen) its more often small scale skirmishes in the town due to religious differences (once again modern Iran is a good example of this, the revolutionary guard often come into conflict with the nomads there due to different interpretations of Islam) or just the fact its in human nature to distrust that which is different, especially if they appear in a large group.
converting this into DF terms could be difficult, but different areas produce different kinds of livestock ect, doable...
And on the farming comment, just because someone chooses not to do something, doesn't mean they can't, in general, farming is much harder work than hunting/raising livestock, but its a safer way to live due to the fact food is far more likely to come. Its all circumstance.