A “race” in UMA is nothing more than a specific TPose and set of DNA converters. For example there are RaceData entries for Male Humans and Female Humans, because they have slightly different TPoses and gender specific DNA converters, despite sharing the same DNA types.
The term “race” can be confusing in UMA. It is used for avatars that share the same base mesh and DNA. For example, if you have a dwarf that is created from the human base mesh simply by changing the dna to be shorter and stockier, then it will still be the same “race” as the human, in UMA terms. This is completely abstract from any game level concept of character races. This is the reason why a human male and a human female are different “races” in UMA, because they have different base meshes.
Creating a new RaceData asset can be done by right-clicking in a folder to bring up the create menu.