The UMA system has been integrated with Atavism allowing users to have UMA characters in their games.
Download the matching Atavism-UMA package for your Atavism version (the major/minor version numbers should be the same) and import the package into your project.
Delete the old Login scene (found in the Assets root folder) and add the new Login and Character Selection scenes to your build settings.
Note: Make sure you don't log in with an account that has non-UMA characters on it. It will break when trying to read in the different data.
A guide on how to set up new UMA character can be found here: Atavism-UMA Customising