Custom MembershipUser/Provider and the CreateUserWizardStep

I have a custom MembershipProvider that only accepts the following parameters in the CreateUser function:
First Name
Last Name
Domain
UserName
Password

I'm attempting to use the built-in ASP.NET create user control.  I've customized the template to ask for First Name, Last Name, and selecting a domain, but none of those variables are passed onto my custom CreateUser function.  These parameters are required to create the user, so I can't use the CreatedUser event (and I'm not using a Profile store since the backend application has no profile store functionality).
How can I go about customizing the CreateUserWizardStep to pass the above parameters to my CreateUser function?
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