unfortunately this resource/widget is not ideal for what you would like to do. I have couple of ideas as an alternative.
1) you can use NetFlow IP groups and create logical views per IP ranges, if that's possible.
2) I assume you want to see "top talkers" for your Nodes within some city. In this case I would crate custom object resource (go Customize Page, search for "custom object" and select "custom object resource"). Add it to the summary page and click "configure custom object resource". In the wizard, select nodes within desired City and add it to the list. Then select "Top XX Conversations" as a the metric that will be displayed for all nodes within specific city. Wizard looks like this :
3) if any options above don't work. try to add "search nodes" resource via Customize page. it's not as elegant as you wanted but it allows you to search the nodes by City or any other custom property. It takes you to the NPM detail page of that node where you can also see or add Netflow data.
let me know if that solves the problem or if not and you really need grouping in "netflow sources" resource.
Thanks Michal, Ill try this out, I just wish there was an easy way to view my netflow sources by address or city or custom properties.