As far as I'm aware, the data only flows one direction, from the 3rd party data partner into facebook, not the other way around.
I don't have a specific use case because its not my industry, but Facebook's approach seems to be to give people tools they know can be misused in creative ways but then frown sternly if they get misused, so it wouldn't be out of character.
From [1]:
"Partner categories are, importantly, only available in specific areas. Acxiom provides data from Australia, France, Germany, the US, the UK, and Japan. CCC Marketing provides data from Japan. Epsilon provides data from the US. Experian provides data from Australia, Brazil, the UK, and the US. Oracle provides data from the UK and the US, and Quantium provides data from Australia."
[1] https://boostlikes.com/blog/2017/08/target-facebook-users-in...