User migration using user import module

I am successfully able to migrate users using user import module. The challenge, I am facing right now is when my .csv file has username column with dot in the column value (ex John.smith or jsmith@test.com), and mapping that column against username from user import module. User import module while updating the database, it stores the username without dot, but replacing dot with space (John smith or jsmith@test com) in the users table. Can anybody provide me solution for this as I have to migrate 17k users and manually doing this will be cumbersome?