Well, while not being an artist myself, I am closely working with another albeit smaller modding community for a game which mainly focuses on 3D which includes quite a lot of hair and so I can maybe already tell...
that the main part of the process is still is to create the 3D assets themselves which is no different from creating them for other games, e.g. one should be able to use Blender and simply export it to UE3 relatively easily if one has the Plugins and settings from the get-go. The XCOM 2 SDK itself contains the Unreal Editor which, if I am not mistaken also allows to work with 3D meshes and textures to a reasonable degree. All that should be left are chores to implement the new customization options which should not be harder than anywhere else since it is linking the files to the menus, make them unavailable to one gender prevent clipping and not end up with a second head instead of hair (but I guess making the menu file will entail that and yes, my other game literary uses a .menu file type, along with .material, .tex and .model).
Oh and since a new mesh always needs a new texture (+all the other stuff), it's obvious that it is more time consuming and kinda harder than only making a new texture with the rest of the strings attached but it should not be too time consuming or too hard, it's probably rather about the demand if more will be made and making it yourself about your own level of skill, talent and dedication.
So there you go, maybe another knows more and feels free to prove or rather just tell me if I am wrong (which I'd appreciate, the more I know the better for me ).