Here’s a temporary solution that will be safe for your case if you are certain that the email addresses of your imported users are valid, and you continue to use the default WordPress login system for your new users. If you ever switch to using a front-end login system that doesn’t validate emails, you must be sure to remove this code. Try copying this function into your theme’s functions.php file:
add_filter( 'discourse_email_verification', 'wpdc_custom_email_verification' );
function wpdc_custom_email_verification() {
return false;
}