When NationBuilder is your email provider it is not possible to set up email forwarding to external accounts. While you can use paths to function as an inbox within your nation, the paths will not generate email notifications. 

If you'd like to have path activities generate email notifications, please add your voice here.

There are free email providers that you can use as an alternative to using NationBuilder as your provider. If you would like to set up an account with one of them externally (and your domain is managed by NationBuilder), you will want to request that they send you your MX records, and then forward these records to [email protected].