How to expose more properties under User class

I am using a custom membership provider.  I want to expose additional properties under User class.  
So in my custom membership provider class I added few Properties (FirstName, LastName, Address, etc) and set their value in ValidateUser().  However back in aspx.cs if I type User. I do not see FirstName, LastName and Address in intellisense. 
Is what I'm attempting to do even possible?  Or do I need to resort to using my own custom user class?  In which case I wouldn't need to use custom membership provider because I can program my custom user class to do everything that custom membership provider can do.  Only thing is I would need to pass this user object in session object.
Any insights from web application pros?
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