Core Data cross storage fetched property
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I'm currently wrapping my head around some problem with Core Data. 
I have one *user* model in its own store that I do not have any control over, as it gets shipped with a framework. A Persistent Store Coordinator, Managed Object Model and Context for this model gets created automatically and cannot be touched. In it, this model has a single *user* entity 

On the other hand, I have a *properties* model with a *properties* entity in it that I have complete control over. In there I store properties for some *user* entities in the other store. Both user and property entities have an *id* attribute similar to a foreign key. 
This model has it's own Persistent Store Cordinator, Managed Object Model and Context.

What I now want is to have the associated *user* entity as an attribute of the *properties* entity so I might be able to bind to key-paths similar to ```myproperty.user.someValueOfTheUserEntity``` (I'm aware that myproperty might be an array when using fetched properties).

However, as cross-store relationships are not supported I thought of using a weak relationship via *Fetched Properties*. That one would just have to match the two corresponding *id* attributes. I have created a Fetched Property for the *user* in Xcode and the required accessors in my *properties* entity's class file (As suggested in other questions, I'm treating the values returned by the Fetched Property as an array). 

However, I'm unable to set a destination entity for the Fetched Property in Xcode, as the target entity resides in a completely different store. Would I also have to define my *user* entity in the *properties* store? If so, how does Core Data know that that entity shall be fetched not from my properties store but from the users store?

Some threads mentioned using configurations for this, but I cannot find any documentation that goes further than mentioning "use configurations for this". 

Can somebody enlighten me on how to set up cross-storage fetched properties? 