Core Data ( Magical Record ) Set same value to an attribute for every entities

Question

I was wondering if it was possible, while using Core Data ( and Magical Record ) to set the same value to a property for every entities.

For instance, let's say I want my Person objects to have the same... T-shirts ( why not .. )

I'm looking for a method like that :

[[Person MR_findAll] setValue:"redTShirt" toAttribute:"tShirt"];

Any idea ?

I don't know if my explanation is really clear...

Thanks in advance !