Is there a way to use the user story import tool and include the parent field? It's not a field in the import format and there's seemingly no way to import multiple items as children of a parent.
The import template in the Rally UI does not support the Parent field. However, if you have the Web Services enabled for your subscription, there is some example code written in Ruby that imports and updates User Stories with the Parent field. Check out this post for more information (need to be a Rally customer to see this post).