DataService search, filter with columns outside fields

How do I filter DataService search with a column which is not part of fields, for example, Person -FirstName is not part of the fields, but need to add a filter for the FirstName.

Litium version: [Version number] 7

Even that first/last name is properties on the person entity they are stored in the field framework and can be queried as all other fields. See constants in Litium.FieldFramework.SystemFieldDefinitionConstants to see all properties that is stored in the field framework.