This has not really been documented by Mythic Entertainment at all, but Warhammer Online supports AddOns just the same as World of Warcraft does, and we're fully supporting it with a Warhammer AddOn section here on Curse.
Those of you in Open Beta may want to check it out -- there are quite a few very cool AddOns already popping up -- with more no doubt on the way as the game inches closer to release. Below are just a few of the "must haves" that players will find extremely useful as you dive into Warhammer Online for the first time.
These are just a few of the AddOns currently available, and more are being added every day, so head over and check them out!