There are two questions here:
1. How do I support custom races in the Portrait Pack Editor tool?
Look for the settings.json file that's in the same directory as the Portrait Pack Editor jar. You should be able to find an array of races. You can add any race you like there.
2. How do I get custom races to use custom portraits automatically?
As far as I can tell, this would require an update to the SfCArics mod to modify how randomized portraits are selected.