Of course, individual authors can always cross over with their own creations from other franchises - the Feledrin gestalt alien in On The Spot is the same species from the DW audio Unregenerate, for example...
You can do that in the UK, but anything we create for Star Trek
belongs to CBS, not us.
My point is that you can import stuff to Trek. Not that's more (in this case) than the reuse of a name and a description.
Exporting it would be more of a problem.
Incidentally, on the matter of non-official crossovers, the best one IMO is with Blakes 7. It may not even be intentional, but if you watch By Any Other Name and then the B7 episode Star One as kind of a two-part story, it works fantastically well...