TypedLists in the designer are generated as Linq queries for Entity Framework. The thing is that the generated projection isn't extensible so you can't 'add' a field to it.
What you could do, but it's not that great, is copy the generated query/projection and augment it to have your custom calculation. Then derive a class from the generated typedlist poco and add a field to that derived class, and fetch the augmented query into instances of the derived class. Not ideal...