As an end user you cannot override an inner join and make it an outer join. You cannot add anything to data that has not already been provided. You can take away, by using filters and conditions, but you simply cannot add.
It is possible for your administrator to add a second join that will not upset any existing reports that already make use of the inner join. The second join would be the outer join named so that it is easily identifiable, perhaps even putting the word OUTER in the join name.
Going forwards, whenever a user tries to create a report between there folders they will be warned that multiple joins exist and prompted to choose which one to use. Once selected you cannot go back and pick the other one. You would have to remove all of the items from the report from one of the folders then start to add them back in. As you add them back in you will once again be prompted and asked which join you should sue.
Hope this helps