To install reporting service feature after SharePoint and project server are installed , you just need to add a new feature (reporting service feature) to the current existing instance by doing the following:
- Just mount the same SQL server media that you are used to installing SQL server at first time or the same version to the database server .
- Run the setup file > Click Installation on the left side of the Wizard and then click New installation or add features to an existing installation.
- Follow the wizard till coming at this step of installation type.
Don't create a new instance , just check add features to an existing instance and select the currently installed instance.
At Feature selection select only reporting service because the database engine is already select at the first SQL installation.
Continue the wizard and select the mode that you need (Native Mode - Integrated Mode)
Now the Reporting services Feature should be installed .
In Native Mode , you can access the reports from Report Manager URL / or Report services URL. and in this case, this URL may be accessible only from DB server , and not accessible outside it based on the current zone.
So to overcome this issue ,
you should install a new instance at SharePoint Server with only reporting service feature ,
then at Reporting Service Configuration Manager , you can configure the Report Server DB to the Main SQL Server Instance ,
Now the Report Manager URL will be accessible and browsable outside the DB server. and the Report Database is hosted at the Main DB Server.