Creating a top level project solution is easy if you have your source control well structured.

Create a new Report Project - this will create an empty or dummy project (which you will later remove)

Add Each of your other projects (assuming you have groups your reports by folder)

Remove the dummy project and save your solution.

This way you will be able to manage the whole reporting solution as a true heirarchy with each project having its own respective publishing parameters.

It also allows other team member to work with individual report projects concurrently.

Hope this helps