Actually it's pretty easy. All you have to have is Twilight of the Arnor.

Launch the... Launcher and select Editors > Improvements. Now name your Improvement ID, and save it. Now you can do all of your modifications to the improvement such as description, images, etc. But under the Bonuses tab, you have 3 options. You are looking for Bonus to Planet, and Bonus to Civ. Under Planet type 7& in the Economic Ability, and under Civ type 1% Economic ability.

Here you go, hope it helps.

-MU

It doesn't? I just made my own test improvement and everything worked fine using the Twilight Editor. You must have done something wrong, check that everything is correct in the actual XML.