Starting from 5.4, you have the ability to view quotas by Sample Broker (if you have more than one broker, with specific quotas). 

The first thing you need to do is to create an agent, with the sample broker name you want, by filling the Broker field (the same name than the one included into your DoExternalPanel links) at the bottom of the Agent properties:

Then when managing quotas, and once you have defined your quota tree, go to the "Web" tab. On the Ribbon > Quota, the "Web Brokers" control is active:

When clicking on it, you can add the required "Web Brokers" (multiple selection allowed):

The chosen brokers will appear as extra columns in the quota tree:

Please note that these brokers will not act in the same way as normal quota variables, but are there to provide a useful progress report on survey completions by broker.