If you want to completely replace the existing one, you'd have ModName\Data\GC2Types.xml. If you just want to add a few new parts, you'd make a xml file with *just* those parts in it, and put it as ModName\Data\ShipComponents\MyNewShipParts.xml, etc. The mods folder has its own structure which the game will create when you use it, and each folder's use should be pretty self-explanatory. It's not the same structure as the Data\English\etc.
Okay... My experiments over the weekend resulted in the game quitting everytime it got ready to finish making the map. So, there's something "bad" with my data files. To that end I have a couple follow-up questions.
According to your post, simple additions to the tech trees, ship components, etc. get put into a specific directory thusly named and not into the Data/English directory. I understand that. However, what sort of XML document syntax is to be used on these? Do I copy the whole header from the master file & use that? Or is the parser going to be satisfied with looking at a "naked" XML entity & be able to slip it into the correct data structure based on which directory it was located in?
The other question maybe completely idiotic, but I'll take the shot for it. Where would modified files that do not corrospond to any of the directorys be placed (i.e., the tooltips.) At the mod's root folder or in the Data/English a la the standard files? Boy, I hope that made any sort of sense.

Thanx for your help with this & once I get my engines finally to load, so too will the Terrans.