I thank you for your answer. I invite to read the copyright statement of WordNet:
Permission to use, copy, modify and distribute this software and database and its documentation for any purpose and without fee or royalty is hereby granted, provided that you agree to comply with the following copyright notice and statements, including the disclaimer, and that the same appear on ALL copies of the software, database and documentation, including modifications that you make for internal use or for distribution.
Everything after "provided that ..." is why you cannot take Wordnet and import it into Wikidata.
But, because WordNet's license (the English one, and about half the global wordnet ones [1]) is still a nice liberal one, what end-users can legally do is use WordNet and Wikidata together in their application.
It is very good to have different sources for the same information, as it allows that same end user to validate data, and discover mistakes, whether human error or deliberate vandalism.
Wikidata should (IMHO) be prioritizing making sure it is as easy as possible to do this. (E.g. producing client libraries to do it; and by documenting equivalences between alternative data sources.)
Darren
[1]: http://globalwordnet.org/resources/wordnets-in-the-world/