This is good to know.  I may have done this subconsiously with one of my personal mod files already (copied another item and changed the effect to taste).

If the item isn't tied to a tech, but you want it in the shop, the K_CityHubs.xml (or F_CityHubs.xml) files designate which items show up at which city levels.  I simply added a tag for said item to the shop, and it appeared.  At the moment, I don't remember what code jujitsu I used to get it to show up in the unit design screen, though...

You may run into a problem if you have an article of clothing (say pants, shirt or dress) tied to the mod though.  These make specific model calls, so implementing them universally gets a little tricky.  I once had a ladies 'wraith shirt' show up on a male model - ugh! Since his torso was bigger than the shirt only parts of the shirt showed through his chest.  Looked like a matter transporter accident or something!

The models are too different. You can't put non-fallen gear on a fallen unit, it looks completely broken.

In my testing of various concepts in the .xml file, I've discovered that Stardock is a little tag happy.

A good example of this is Base Hit Points.  95% of the units in the game list this as 0 (so HP are calculated soley based on con/level), and the CoreUnitStats.xml file defines this value at 0 already.  I've successfully removed said tags from everywhere else I've found it (where said value was 0 already), and the CoreUnitStats.xml value is used.  Changing said value in CoreUnitStats.xml to 5, for example, will add 5 to the hit points of all units where said deletion was made.

Probably in the 'early days' of development, it was necessary to define all of these values, but now that things are more streamlined, removing redundant code would probably be more efficient.  Note that .xml files aren't that big to begin with, so I don't consider this a burning issue.