I guess your question is "how to use MembershipUser/Membership and Role Provider in nHibernate".
First of all, if you don't want to reinvent the wheel, I'd suggest to remove the MembershipUser property from your class and just put a reference to some identifier. Otherwise it is getting more complex. Later you can convert/retrieve the MembershipUser from your MembershipProvider...
There is one good article on codeplex describing how to implement a custom membership provider with nHibernate http://www.codeproject.com/Articles/55174/Custom-Fluent-Nhibernate-Membership-and-Role-Provi