Just to be clear here there is a difference between races and factions. Race is what determines what basic units you use and after that faction is what determines your faction traits, colors, and crests.

If your trying to add Juggernauts to a new race it can be confusing because of the Train Juggernauts ability some Sovereigns have. This does nothing at all, it exists simply to inform you that the race can train Juggernauts. It does not unlock them. If you look at the Juggernaut xml you will see their prerequisite is tied directly to the Trog race. Thus the easiest way to add Juggernauts to a new race would be to copy and create a new Juggernaut with a different race prerequisite.