Migrate user password hash
Since you don't know the password, only the hash, you can't migrate it directly. (MD5 is weak, but not that weak.) The whole purpose of hashing is that the password should be unknown to anyone except the user. There are 2 solutions to this.


Reset all your users password and send a one-time-login password to them once they are migrated - as already suggested in the comments above. 
Write custom code that rehashes the password at login: "If the password is hashed with MD5, rehash it and store the hash when the user (successfully) logs in." This works because at login you have the user's password because they just entered it. And you know which hashing algorithm is used from the stored hash.Initially, you migrate the users with the MD5 password hash. 


It turns out that Drupal does rehashing automatically (see UserAuth::authenticate()), but not for MD5 unless you modify the  existing hash slightly by prefixing it with an "U". (Thanks, d70rr3s and  Souvik Das.) See also user_update_7000.