Warhammer includes a comprehensive modification API which allows addon authors to create interface modifications. These interface modifications, typically called 'AddOns', can be created for all sorts of purposes such as modifying the default inventory screen, keeping track of complex game mechanics like threat, or cleaning up your inventory.
Currently addons are available for Warhammer at Curse Gaming  and probably at other locations.
AddOns can be installed by navigating to your Warhammer Online install directory, which is C:\program files\Electronic Arts\Warhammer by default. Once there you will probably need to create the folder "Interface" and then inside that folder another one named "AddOns".
Your directory structure will look like this now:
If these directories already existed then don't worry just continue on from here.
Next you need to open up the zip file containing your new addon and extract the folder inside the zip into this AddOns directory.
Make sure you are only downloading zip files for your addons. In other online games which support addons it is common for virus writers and keylogger writers to hide their software in addon installers. Try to make sure you only download addons from websites that you trust and only when they are in zip format.
Currently, EA Mythic has not released any documentation on AddOn creation. All information about AddOn creation is gathered via reverse engineering the current default interface contained in the interface.myp file. As such, all data regarding AddOn creation is subject to change and is in no way guaranteed to be accurate.
There are three different kinds of files which make up an AddOn. There are MOD Files, XML files, and LUA files. The MOD file defines what the mod contains and provides Warhammer with some basic overarching information regarding how the AddOn will behave. Secondly, the XML files generally define how the interface mod will look. Last, LUA files contain the code and logic behind the AddOn.
For more detailed information on AddOn creation, visit TheWarWiki