Yes. There's example configuration files for adding new background options in the ERA\RMFRP\Configuration\BackgroundOptions directory. Use an editor and follow the XML format, and when you restart ERA the new option will be available when you create a character.
Or if you want to add it to an existing character, find the character definition in ERA\RMFRP\Characters directory, and you'll find the place in the file where the other background options have been included, and just type them in as a free form text description. You can do this without creating a background option file.