Add additional field to collection

Hi,

we created a workflow to display stores that are in the same city of the profile.

We created an enrichment that creates a N relationship with the table store based on the city. We want to display 6 stores and sort them by the zipcode using the formula [zipcode - profile/zipcode] (zipcode are number so is correct for our country).

The problem occurs when we want to create this formula in the collection. It doesn't show up the zipcode of the profile. How can we sort the collection with the formula?