This is currently not possible within the Atlas UI.
However, there is a way to create such a report if you're prepared to use another (native) client that accesses the underlying Hub data directly (StarTeam CPC).
It involves several steps to initially prepare the data filter - but afterwards you'd have a flexible, adaptable way to generate this and comparable reports.
Are familiar with the CPC and/or would using it be an applicable workaround for you?
In that case we can provide you with the steps on how to get this data using the CPC...
OK, I'll leave out the parts how to get the CPC and how to access Atlas projects with it...
Within the CPC, the steps are as follows:
1) Open project in CPC
2) Go to type "Topic" and switch to "List Display"
3) Create a query (e.g. right-click on list header row) to only show open discussions:
-> "Parent Object ID" = -1 AND "Status" = "Active"
4) In Menu Topic | Reports run the "Classic" report "Links" with scope "Select All"
5) Note the "Item ID" in the generated report, decrease the ID value by one and enter this in the "Find Requirements" field in Atlas