You can customize the joins used between different data sources by clicking on Configuration -> Customize Joins within the report editor. When you first come into the Customize Joins screen, you'll be presented with the following:

By default, the report uses the joins configured at the data source level. If you want to customize the joins for a particular report, you can click on the toggle for "Should this report use custom joins?"

Once you've toggled this flag, you can drag joins to reorder, edit joins, add new joins, or remove existing joins. Joins used in the course of a report run are highlighted in green. If you wish to restore the report to using the default data source joins by clicking on Revert to Data Source Default and saving.

If your data source combines multiple combined data sources (for example, if you're joining Basecamp to Highrise), you can