You can edit existing factions by editing the XML file that holds the faction.  For custom factions, this can be found (on Windows) in

          *\My Documents\My Games\FallenEnchantress\Races

and will be titled Faction_FactionName.xml.

XML files can be edited using any text editor, such as MS Word, Notepad, Wordpad, etc.  Unfortunately, as far as I know this is the only current method of editing any faction, as the in-game faction editor doesn't include a 'load faction' option, and I am not aware of any current 3rd party utility intended for editing factions.

The vanilla factions can be found in the installation directory under data/English/CoreRaceConfigs.xml

Unfortunately getting the Stormworld races and faction abilities for custom traits is a bit of a lost cause, because I had to structure the XML differently. In short, each faction also belongs to a race - Altar faction belongs to the race of Men for example. In the vanilla game, all the faction abilities exist on the faction level. For the Stormworld factions, many of their special things are in the race level.