The generic insults used by most enemies (and yourself) in the game are found in <Install Dir>\Data\Dialog\gerenicInsult00.xml. While you could just edit this file and add some new lines, that runs the risk of getting overwritten when the game updates. The "correct" way of doing this is to add a file to the Mod folder of the game. (See here for a thread from the game dev explaining this, as part of how to create a skybox mod.)

To begin with, locate the game directory - it's easiest to use the 'Browse Local Files' option from Steam. Then, you'll need to go create a subdirectory under the 'Mod'directory: 'Mod\Data\Dialog'. (The same thing applies to modding other parts of the game as well - it should be the same as the location under the game's "Data" dir, but placed under the "Mod" dir instead.)

In this directory (<Install Dir>\Mod\Data\Dialog) you'll need to create a file called (whatever).xml - where '(whatever)' can be whatever you want to call it. You should probably pick something like "extra_insults.xml" so you can tell it apart if you add more files to customise other game dialog.

Inside the file, you just need to add this:
Where it says you can use whatever ID you like. The built-in insults are ID'd as "civieInsult00", but it doesn't seem to make a difference if you re-use that ID or call it something else. (For most object types - such as ships - reusing the existing ID will replace the regular object with your mod, but this doesn't seem to be the case with dialog files.)

You can edit the "<dialog ...>" lines, and add more of them, to use whatever insults you want.

And voila - launch the game, and you have your new insult messages added to the existing ones.