This feature is not available in Source (public version).
Source allows you to import scenarios from a project file. This is useful when there are two project files that have related scenarios or scenarios that you wish to bundle together into a single project. This feature is works well with Copy \ Paste nodes and links when building a single scenario form a number of smaller scenarios.
If you have an open project, you can import a scenario by choosing File » ImportScenario. This allows you to select a project, then opens the Import Scenario dialog (shown in Figure 6). Follow the instructions shown in this figure to import a scenario. Note this when loading a project (Step 1).
You can import as many scenarios at a time as are in the loaded project. Click once to select; click again to deselect.