This plugin automaticaly links all the twitter users that you mention on your posts or pages, using a filter. You just have to make sure mentioning users using @ sign.

1.6

• Contributors

1.5

• Tested up to 4.4
• Banner and icon added

0.5

• Initial release

1. Upload 'link-twitter-users.php' to the '/wp-content/plugins/' directory
2. Activate the plugin through the 'Plugins' menu in WordPress
3. Enjoy!