First export the account privilege info to a file:

SELECT * FROM user where User LIKE "username_%" INTO OUTFILE '/tmp/username_users.txt';

SELECT * FROM db where User LIKE "username_%" INTO OUTFILE '/tmp/username_db.txt';

Copy these files to the new server and import them into the "mysql" database.

mysql mysql
LOAD DATA INFILE '/tmp/username_users.txt' [IGNORE|REPLACE] INTO TABLE user
LOAD DATA INFILE '/tmp/username_db.txt' [IGNORE|REPLACE] INTO TABLE db
FLUSH PRIVILEGES