Courgettes aren't really all that well known to Americans, I just happen to remember reading about 'em on english.SE. It would be silly to retag with zucchini or summer squash, but can we add a tag synonym without another question even using that tag?
No, the tag has to actually exist somewhere at some point in time for the system to accept it. After all, there's rarely any need to set up a synonym for a term that isn't used.
Moreover, a synonym simply causes the system to substitute one tag for another when editing (and searching), so unless a tag becomes pervasive, it's probably just as useful to simply edit in its replacement manually.
Of course, you can suggest any tag synonyms you feel might be helpful here on Meta, and we'll implement them (unless someone can think of a good reason for not doing so, as with cilantro -> parsely)