public class FrontEndUser : DBEntity<int>
    {
        public virtual string Name { get; set; }
        public virtual string LastName { get; set; }
        public virtual string DisplayName { get; set; }
        public virtual string Email { get; set; }
        public virtual DateTime DateOfBirth { get; set; }
        public virtual string Address { get; set; }
        public virtual string City { get; set; }
        public virtual string State { get; set; }
        public virtual string ZipCode { get; set; }
        public virtual DateTime CreationDate { get; set; }
        public virtual bool Active { get; set; }
        public virtual IList<SecurableUserLevel> UserLevels { get; set; }
        public virtual IList<SubscriptionProduct> Subscriptions { get; set; }
        public virtual IList<FrontEndUserLogin> Logins { get; set; }   
     
    }

public class SecurableUserLevel : DBEntity<int>
    {
        public static readonly int LEVEL_NOT_LOGGED = 1;
        public static readonly int LEVEL_BASIC = 2;

        public virtual string Name { get; set; }
    }
}

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="AdminNewsCms.BusinessObjects" namespace="AdminNewsCms.BusinessObjects.Security" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:nhibernate-mapping-2.2 ..\nhibernate-mapping.xsd">
  <class name="FrontEndUser" table="FrontEndUsers">
    <id name="Identity" column="FrontEndUserId">
      <generator class="identity"/>
    </id>
    <property name="Name"/>
    <property name="LastName"/>
    <property name="DisplayName"/>
    <property name="Email"/>
    <property name="DateOfBirth"/>
    <property name="Address"/>
    <property name="City"/>
    <property name="State"/>
    <property name="ZipCode"/>
    <property name="CreationDate"/>
    <property name="Active"/>

    <bag name="UserLevels" table="FrontEndUsers_Securable_UserLevels" fetch="join">
      <key column="FrontEndUserId"/>
      <many-to-many column="SecurableUserLevelId" class="SecurableUserLevel" />
    </bag>

    <bag name="Subscriptions" table="FrontEndUserSubscriptions" inverse="true" cascade="all-delete-orphan">
      <key column="FrontEndUserId"/>
      <many-to-many column="SubscriptionProductId" class="SubscriptionProduct"/>
    </bag>

    <bag name="Logins" table="FrontEndUserLogins" inverse="true" cascade="all-delete-orphan">
      <key column="FrontEndUserId"/>
      <one-to-many class="FrontEndUserLogin"/>
    </bag>

  </class>
</hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="AdminNewsCms.BusinessObjects" namespace="AdminNewsCms.BusinessObjects.Security" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:nhibernate-mapping-2.2 ..\nhibernate-mapping.xsd">
  <class name="SecurableUserLevel" table="Securable_UserLevels">
    <id name="Identity" column="SecurableUserLevelId">
      <generator class="assigned"/>
    </id>
    <property name="Name" column="Name"/>
   </class>
</hibernate-mapping>

            FrontEndUser frontEndUser = new FrontEndUser();
            frontEndUser.LastName = lastName;
            frontEndUser.Name = firstName;
            frontEndUser.DisplayName = displayName;
            frontEndUser.Email = email;
            frontEndUser.DateOfBirth = dateOfBirth;
            frontEndUser.Address = address;
            //ETC ETC ETC... next lines are the important ones
            frontEndUser.UserLevels = new List<SecurableUserLevel>();
            SecurableUserLevel userLevel = SecurableUserLevelService.GetSecurableUserLevel(SecurableUserLevel.LEVEL_NOT_LOGGED);
            frontEndUser.UserLevels.Add(userLevel);

            FrontEndUserService.Save(frontEndUser);