This was discussed extensively here: https://www.pantheonmmo.com/content/forums/topic/2628/third-party-add-ons
The short answer is no, the plan is that there won't be add-ons. They want the focus to be on the world and not your UI. They also want to avoid tools that might allow you to get an advantage over other people. You'll be able to reskin and resize/move UI elements, but no actual add-ons.
10.6 Will we be able to customize our own UI in-game without add-ons?
Yes, the UI will be customizable and skinnable. We will likely avoid, however, true scripting languages that might allow a player to create macros that automate gameplay.
To summarize that thread Shuck linked, a quote from Aradune: "The plan is a customizable UI and then being very careful about the data we send to the client so that third party apps that intercept that data won't be very useful."
Also see these clips:
00:39:07 - Third-party mods
