[SalesForce] Visual force error message in a new page

I'm creating a custom VF page for the registration of new users.

Page: simple form to get the user's email.

Controller:

public with sharing class SiteRegisterController {

    public SiteRegisterController () {
    }


public String username {get; set;}
public String email {get; set;}
public String password {get; set;}

    public PageReference registerUser() {
       user u = [select id,username from User where username = :email];
           if (u!=null) {
                  //ADD HERE CODE FOR ERROR MESSAGE IN THE PAGE FORGOTPASSWORD
                  PageReference page = System.Page.ForgotPassword;
                page.setRedirect(true);
                return page;
            }
            else {
                PageReference page = new PageReference('http://registration.com/');
                page.setRedirect(true);
                return page;
            }

        return null;
    }
}

If the user is already registered i would like also add a message "You are already registered, Did you forget your password?" on the new page System.Page.ForgotPassword;.
I know how to add an eror message in the current page

ApexPages.Message myMsg = new ApexPages.Message(ApexPages.Severity.ERROR,'Error Message.');
ApexPages.addMessage(myMsg); 

but is it also possible add it in a new page(forgotpassword) from the controller of the current page?
Of course in the page forgot password there is a tag <apex:pageMessages id="error"/>

Thanks in advantage for any advice.

BR.