Unfortunately not, in a self-registration you can't capture anything other than username/password (which can be either mobile phone or email).
The Cloud Guest Splash page profiles don't support custom content either for you to place this field yourself.
If you have an Aruba partner login, please check for or raise an idea on the Aruba Innovation Zone (AIZ): http://innovate.arubanetworks.com/
Otherwise, reach out to your Aruba partner/account team and discuss the point wiht them.