Just generating an email address won’t be enough. You should definitely disable emails for those users as I’m doing for anonymous users. You could either do this in the base importer or just modify the phpBB importer so that it handles users with missing email addresses similar to anonymous users.
BTW: It’s a good idea to use the .invalid TLD for generated email addresses (just in case something goes wrong and Discourse starts to send emails to those users). I’m using the following for anonymous users:
"anonymous_#{SecureRandom.hex}@no-email.invalid"