You wouldn't need to make a new faction XML, no, that's just for ease of finding specific units.

To start with, you'll need to locate where units are stored - this will almost always be the files in Data/XML/Units. I would also suggest downloading either Visual Studio or Notepad++ before going further, just for ease of use on your end.

After that, you'll need to find the unit you want to add; generally they'll be in the Militia_x and either Other_x (for FOTR) or Unit_x (For TR). For example, the Interceptor is in Units_Corvettes whilst the various Mercenary units are in Militia_Ground_Units.

Regardless which unit you pick, the method for making it buildable is going to be the same;

Firstly, add in the faction you want to build them to the <Affiliation> Entry; you can look at other units for examples on what you need to type.

Secondly, add in the shipyards associated with said faction; again, looking at other units for examples can save you a lot of hassle.

Thirdly, and sometimes optionally, you will need to disable anything that locks the unit from being buildable; the most common cases for Independent Force units is tech level being set to 99 - simply set that to 0 - and the Build_Initially_Locked tag. Units associated with other factions might have other locks associated with them that you'll need to find and disable.

Of course, when Thrawn's Revenge updates to 3.5, Custom GC will allow you to build various independent forces units without needing to mess with the files.

Edit: Reworded some stuff as I was using FoTR as a reference point originally.

Really appreciate the reply!

Just want to chime back in here in case anyone reads this in the future - I can confirm that this XML editing works and is all you need to do to enable these units