Copying the files is only half.
*****Edit:
I only just noticed that the file names duplicate the original style.
I had changed the names to the actual ship name, so s99_Huge_1.x was renamed to s99_Basestar.x. Just made more sense to me at the time.
S99_ is the prefix to allow all races to use the hull.
But I see a conflict arising if there is an s0_Huge_1.x and an s99_Huge_1.x if the player is not using a custom race. If the player is Terran the game may always use s0_ over s_99, and so the new hulls would never show up unless they are renamed and also have unique Hull entries in GC2Types.xml.
(If you rename the .x files - do NOT rename the .png files)
*****
The other half is specifying the hulls in GC2Types.xml, or in a .xml file in the current Mods\(yourmod)\Data\ShipComponents folder.
It is always better to copy the files into the gfx\models (and gfx\thumbnails if thumbnails are provided) folder under the mods folder. It keeps your base install a lot cleaner, and it may be necessary if you are specifying the hulls in the above mods location.
I have a separate file for the Battlestar hulls - 'Battlestar Hulls.xml' - located in C:\Program Files\Stardock\TotalGaming\GalCiv2\Mods\KHSM\Data\ShipComponents.
I also have a 'Vulcan Hulls.xml', 'Federation Hulls.xml', 'Asgard Hulls.xml', ...
And example entry for the Basestar (in either the main GC2Types.xml or a separate file in mods...) would be: ( < and > were changed to [ and ] for posting)
[Hull Name="BaseStar"]
[DisplayName]BaseStar[/DisplayName]
[Description]Cylon BaseStar[/Description]
[Cost]160[/Cost]
[Size]80[/Size]
[SensorRange]4[/SensorRange]
[HP]50[/HP]
[Model]Basestar[/Model]
[Thumbnail]Basestar[/Thumbnail]
[Speed]1[/Speed]
[Category]Huge[/Category]
[Logistics]8[/Logistics]
[Tech_Requirement]Massive Scale Building[/Tech_Requirement]
[/Hull]
If you create a separate file for the hulls, make sure you get the file structure right:
[?xml version="1.0" encoding="UTF-8" standalone="yes" ?]
[GC2]
[DataChecksum NoParse="1"]
[Ignore]DisplayName,Description,Model,Thumbnail,Animation[/Ignore]
[/DataChecksum]
[Hull Name="BaseStar"]
[DisplayName]BaseStar[/DisplayName]
[Description]Cylon BaseStar[/Description]
[Cost]160[/Cost]
[Size]80[/Size]
[SensorRange]4[/SensorRange]
[HP]50[/HP]
[Model]Basestar[/Model]
[Thumbnail]Basestar[/Thumbnail]
[Speed]1[/Speed]
[Category]Huge[/Category]
[Logistics]8[/Logistics]
[Tech_Requirement]Massive Scale Building[/Tech_Requirement]
[/Hull]
[/GC2]
That would be the complete code for 'Battlestar Hulls', minus all the other hull entries.