I havent' gotten going full tilt .xml modding yet, but seeing as the game has Buildings that do this already, search the Dir's related to those buildings, find the related .XML (if available) and use it/those as a reference guide to your new Unique building.

A few tweaks to an existing .xml file and/or copy/paste from the old to your new units .xml file should do the trick.

I will have a look tonight to see if the files can be found. (if someone else doesn't post it up before that)