I couldn't find any information (googled it) about SDK or adding custom "addons". I noticed that this game is made with Torque 3D and it has similarities to Unity 3D.
I am interested to create own addon to the game is it possible? I mean you may open a UI window in game like like inventory or crafting which shows you data.
With community help this game could develop even faster. Its like a WordPress with create plugin creators (of course it opens up for bad addons too).
I personally think that Your Own development has stopped because of MMO.