Need direction: Passing form variables

Can I pass variables I collected from a form that is stored on my website, and autofill another form on another website I do not have access to?

Example:” has a sign up form. Fields collected include FName, LName, Email, ZipCode. After visitor signs up, they are offered two more websites to sign up to. These websites could be opened in an iframe or new window. I would like to prepopulate their form fields with the information collected on my database. I have no access to the other websites.

Is this possible? If so please point me in the direction I need to go to accomplish this.

Thank you in advance for any help.

If that were possible it would be through the use of cross-site scripting, otherwise known of as XSS, and is something up with which we shall not put.