You'd be better off asking the Scenarios, Maps, and Mods sub-forum.
There's no way to assign this directly, but you can use #copystats to copy a unit with already as a baseline, and then modify the other attributes. For instance, the
Son of the Dragon King has underwater transport, and not a lot of other attributes like being a seducer or granting luck that you might not want to copy.
You can't mod an item onto a unit (except via map commands.)