Well, first thing is to be able to access the XML files of the game. Best way I'm aware of is to go to your 'Mods' folder within the main FoC directory, create a new folder, place a 'data' folder within that, and then create an 'xml' folder within the said 'data' folder. You can find a template for folder layout in any mod you download. To access the modded version of the game, simply copy/paste a shortcut of the FoC .exe somewhere and change its target line in properties like so:
C:\Program Files (x86)\LucasArts\Star Wars Empire at War Forces of Corruption\swfoc.exe
"C:\Program Files (x86)\LucasArts\Star Wars Empire at War Forces of Corruption\swfoc.exe" MODPATH=Mods\Mod's_name
Just a matter of quotes and that MODPATH bit. "Start in" line stays the same
So... In order to edit this turret business, you need to get the game's original XMLs and place them in that XML folder you made. The only ones you actually need are those with "GROUNDBUILDABLE" in the name.
I'll give an example of how you would code in the Empire's torpedo turret version or whatever it is you want.
Btw, this is the very simple version, as making it complicated might confuse things. Remember you don't really need to back anything up as you will not be editing the actual game files, only copies.
So, to keep the Empire build voice, open up "GROUNDBUILDABLESEMPIRE.xml". Scrolling down, you will see a list of statistics for the Empire's buildable turrets.
What you need to do is simply create a new turret
I'll show you what it will look like
It's that simple. Paste that somewhere in the file I just mentioned, somewhere before and after two other turrets, i.e. not within another turret. If you did the latter the game would crash due to Syntax error >.<!
Once you've done that, last stage is to code in that it should be buildable in the MP (skirmish, too) and campaign (GC as well).
Find where it says <Tactical_Buildable_Objects_Campaign>... and <Tactical_Buildable_Objects_Multiplayer>, and add in Empire_Proton_Turret somewhere in that list, separated from the other turrets by a comma and a space as has been done with all of the others. Add it into both of those lists so that it can be built for all game modes across the board. Get rid of the sensor node if you like, it will be obvious which one it is from the name.
Once you've done that, save the file and you're done there. Last stage is to open up "GROUNDBUILDABLESSKIRMISH.xml" and find a similar list of buildable turrets. In the list (again only under <Tactical_Buildable_Objects_Campaign> and <Tactical_Buildable_Objects_Multiplayer>) in the 'paragraph' under "Empire," add the Empire_Proton_Turret as you did before. Likewise, remove the sensor node. Again, try and stick exactly to the format the rest of the turrets are in.
Mind, you might be able to keep the sensor node if that works. I'm not sure if more than 6 options will show up, but it might be worth a try.
Save that file and you're done.
Then launch the .exe shortcut for this modification you've made and hopefully it'll work :P. I haven't tried it but I might do, and if there's any hiccup with this method I find I'll come back and tweak it.
If you want further help, just say, and I can explain.
Using this method for all factions and all turrets is possible, just repeat what you've done here but call the turret something different, and make sure that in the 'variant' parenthesis (<GroundBuildable Name="...) you specify its <Affiliation> </Affiliation>
Also, I forgot to mention this before, but you can call the turret whatever you like.
Oh and in case you didn't know already, you can open .xml files with notepad or wordpad. I prefer Notepad++ though, google it. Makes things lots easier with colouration etc.