No, email addresses are not needed for user authentication.
I'm not an impex expert but you may be able to get away with adding a new column to your spreadsheet for emails and using a generic email addy for everyone.
user@yourforum.com
edit: I think impex needs email addresses because users will need to reset their passwords IF the password was encrypted in the source system. I dont think you have this issue with unencrypted / hashed passwords though.