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I don't recall seeing a Borg cube keycaps, might be a neat idea!
As far as copyright goes, there are all sorts of keycaps made based on copyright images.
It is ultimately up to the copyright holder, but generally it seems that the big companies don't worry that much about a very limited fan-made replica.
If you started mass-producing them and/or charging a lot of money and/or trying to associate your product with the original concept (Star Trek series, in this case) then they would probably send you a C&D letter.