Well, if I were to do that, it wouldn't be in this mod.
I have done two things in my overhaul mod that technically do what you want here. Firstly, I can easily create clans to be used in custom battles. Its really just a new row in three faction db files, some text entries, some new crests and a banner. Easy stuff. The more interesting thing I did was simply swap out aforementioned files of a pre-existing campaign clan with files I created from scratch that lead the player to believe there is an entirely new clan available.
Adding an entirely custom clan to the campaign is just... well let's just say I don't think anyone has actually done it for the warscape startpos.esf files
.