If you want to store anything in a property of an entity, you can declare it as Object or dynamic; as long as you set it to a value of a serializable type, as supported by BinaryFormatter, you’re good! You can even change the value afterwards to a different type, and it will be saved.
You map it like this, in mapping by code:
NHibernate will return the same object type and all of its contents anytime you retrieve it.
Try that in other persistence frameworks!