nhibernate many-to-many mapping - additional column in the mapping table?

I have following mapping definitions:

  <class name="Role" table="Role" optimistic-lock="version" >

    <id name="Id" type="Int32" unsaved-value="0" >
      <generator class="native" />
    </id>

    <property name="RoleName" type="String(40)" not-null="true" />

    <bag name="UsersInRole" generic="true" lazy="true" cascade="all" table="UserRoles" >
      <key  column="RoleId" />
      <many-to-many column="UserId" class="SystemUser, Domain"/>
    </bag>

and

<id name="Id" type="Int32" unsaved-value="0" >
  <generator class="native" />
</id>
<property name="UserName" type="String(40)" not-null="true" unique="true" />

This mapping generates mapping table UserRoles, which has two columns - RoleId and UserId.

However, I'd like to add extra attributes to that relation - i.e. some enum values defining state of the relation as well as effective start & end dates.

Is it possible to do in nhibernate or do I need to add additional class here and change relation m-to-m into 2 relations [user] 1-to-m [user_role] m-to-1 [role] ?