Simple hack for PHPBB3 --> use email (first part) instead of manual username.

gistfile1.php

// Edit file includes/ucp/ucp_register.php

$emailParts = explode("@", strtolower(request_var('email', '')));

if(count($emailParts)>1) {
    $userName = $emailParts[0] . "-" . substr($emailParts[1], 0,3);
}else {
    $userName = utf8_normalize_nfc(request_var('username', '', true));
}

$data = array(
    // 'username'           => utf8_normalize_nfc(request_var('username', '', true)),
    'username'           => $userName,
    'new_password'      => request_var('new_password', '', true),
    'password_confirm'  => request_var('password_confirm', '', true),
    'email'             => strtolower(request_var('email', '')),
    'email_confirm'     => strtolower(request_var('email_confirm', '')),
    'lang'              => basename(request_var('lang', $user->lang_name)),
    'tz'                => request_var('tz', (float) $timezone)
);

// You can also edit cache/tpl_{$used_style}_ucp_register.html.php for hidding some fields.