The alpha has always been moddable. It works the same way it did in GalCiv3. The full game is supposed to have something similar to the workshop in Steam so that mods can be used no matter where the game was purchased.
Editing the xml files where they exist doesn't work. A folder for a mod must be created in the Mods folder, the xml files need to be copied into correct sub-folders in the folder for the mod, then those xml files can be edited. Mods must be enabled in the game options.
There is a ModdingReadme.txt in the Mod directory that explains it more, but it refers to folder locations for the original GalCiv3, so adjust that to match what is correct for GalCiv4.