To clarify the camp-to-city conversion process:

In convertible camp type settlements one can build either HERDS or FARMS (or a combination of these two buildings, but that is not advisable if one aims at converting the region eventually because at higher levels these buildings become mutually exclusive). HERDS are typically nomadic building and best used in regions that are supposed to remain nomadic throughout the game. On the other hand the FARMS in nomadic regions slowly increase the "Eastern Tribal" culture in the region, thereby reducing the "Nomadic" one and allowing higher level of these FARMS to be built.

Once the settled culture level raises far enough and the highest level of farms is built (it may require to destroy the competing HERDS building) one can then build the usual M2TW camp-to-city conversion building.