If you look at app-route-map.js.es6, you will see that the routes for the activity types (topics, replies, bookmars, etc.) are nested inside the user.userActivity route, so you could create a route map:
then create a template user-activity-new-stream.hbs (this will be displayed inside the outlet you mentioned before) and link to that route in your user-activity-bottom connector.
Also, in case you wonder, the activity types routes use the user-stream and the stream-item components to populate the right side section.