You can now add DLC and create Achievement lists specifically for DLC.
It works extremely similar to how adding achievements, guides and wiki pages already works. From the game home page, you can create a "DLC List" or "DLC Achievements List". From the newly created list page, you can submit DLC or DLC Achievements.
Each item of DLC should have it's own Achievements list. For example, check out the following GTA DLC and DLC Achievement pages