I don't think SSAS allows you to add string fields as measures. You might want to try adding the field as a dimension. Then you can access it from the drilldown query as:
RETURN
([DIMENSION].[$DIMENSION.FIELD] AS [ALIAS])
In the Dimension structure, you can also set the AttributeHierarchyVisible field to 'False' if you don't want users to see this field in the list.
Hope this helps,
Manny