XCOM: Enemy Unknown, but designer Garth DeAngelis has stated that modding tools aren’t “on the table currently.” He did offer a little hope saying that they may come "someday".
According to DeAngelis, “Technically, sure, there’s a lot of support for (Unreal Development Kit) and modding and things like that. And, if we were to do it, we’d want to do it right. We’d provide the proper level of tools and support and that takes a little more planning and time to get that right and we’re not on that right now.”
There are some mods already available at various fan sites, but there are no dedicated tools to make XCOM content as rich and varied as, say, The Elder Scrolls V: Skyrim.
The upcoming major DLC pack Enemy Within will be available for XCOM: Enemy Unknown on the 15th November.