Adding civs to a mod question

[colonialfan]

Does anyone know how to add civs to a pre-existing mod, and if so what files do I need to alter and what those files are? Essentially, all I'm looking to do is to add the Portuguese, Danish and Swedish civs to the standard four other civs found with the Slavery Market Mod. If anyone could give me some tips on how to do this that would be great.