It's not something you can import. As it isn't a downloadable list as such.
Many players keep a written (or typed/digital) record of people who have killed them or wronged them in some way. Those players tend to pass that information on to anyone who will ask, so if you get instanced with a player from their list you could put the player on block - even if they have not done anything to you but just from the recommendation from the first person.
Though going around asking who to block is one thing, the fact you have to be instanced with them so you can block them is another

(unless you friend them first, then you can move someone from friend to block in the game menu should they accept the friend request)