private static ISessionFactory Configure()
{
    if (_factory != null)
        return _factory;

    var configuration = new Configuration().Configure();

        // I could set my assembly of mapping here, but it's on our internal framework
    var fluentConfiguration = Fluently.Configure(configuration)
        //.Mappings(c => c.FluentMappings.AddFromAssembly(typeof(ProductMap)))
        .BuildConfiguration();

    _factory = fluentConfiguration.BuildSessionFactory();

    return _factory;
}

<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
    <session-factory>
        <!-- other configs here...-->
        <mapping assembly="MyApplication.Data.Mapping" />
    </session-factory>
</hibernate-configuration>

private static ISessionFactory Configure()
{
    if (_factory != null)
        return _factory;

    var configuration = new Configuration().Configure();

    foreach(var alteration in alterations)
    {
        alteration.AddTo(configuration);
    }

    _factory = fluentConfiguration.BuildSessionFactory();

    return _factory;
}

// in your alteration
Configuration AddTo(Configuration config)
{
    return Fluently.Configure(config)
               .Mappings(c => c.FluentMappings.AddFromAssembly(typeof(ProductMap)))
               .BuildConfiguration();
}