It is not possible without a custom developed solution. I do not recommend modifying any mojoPortal code.
Basically in the current implementation we have to ways users may sign up for newsletters. Authenticated users can sign up from the My Account page or if they use the newsletter form it will still attach it to their user profile if they are authenticated.
For authenticated users, we do have a custom user profile system where you could add extra properties required for registration on the site, but these do not show up in the separate email subscription form because it is designed also for unauthenticated users to sign up for newsletters in which case we only capture the email address and the user does not become a site member by subscribing to the newsletter. But if the user later does come back and register on the site a user will be created and his existing newsletter subscription will then be associated with his user account.