Yes is possible but we need some information for give you a simply solution.
- have you only a domain hosted or you have some service?
- your domain service use a cpanel or what?
If you have cpanel is verry simply trough cpanel set your hown third domain to your dinamic ip connection.
i mean (arduino.yourdomain.com) is verry simply to redirect to arduino.yourdomain.com dinamic ip from your connetion( e.g. AOL connection)