@xertra11 Yea I'm almost certain it is possible to have units from different factions in your own custom Orbat faction. Once you have created your faction (setting its name and classname and side and such), go into the Unit Editor of Orbat, and click on "new" and a window should appear. What you want to look for in the "create unit" window is the "unit type" part. There you will basically select on which unit your new one is based. You can base your new unit on pretty much every other unit that are in game (be it vanilla ones or units from the mods that you have loaded at that time). Though, in the "Ownership" part of the "create unit" window, make sure you set the correct Side and your new faction, this will define to which Side and faction the new unit will be a part of.
Not sure if I explained this clearly, but the Orbat is fairly easy to understand how it works, anyhow, here's a tutorial that might help you more (that's also linked in the Orbat pages of the Alive wiki If I recall correctly):