Hi Victordpc,
Is the existing project a BI project? Does the solution that you want to add existing project to is a blank solution or there has other type project? Please provide some screenshots and describe the detailed steps that you do to help us reproduce your problem.
Please try with following steps to confirm whether this problem is related to Visual Studio or your solution.
1.Create a new blank solution with Visual Studio 2013.
2.Right-Click and choose "Add -> Existing Project".
3.If it still has the same problem, please try with below steps to modify your Visual Studio 2013.
(1) Open Control Panel to find the Visual Studio 2013
(2)Select Visual Studio 2013 and right-click to choose "Change"
(3)Click the "Modify" button.
(4) Check the "Microsoft SQL Server Data Tools".
(5) "Update" Visual Studio 2013.
4.If the new solution can add the BI project, this problem is related to the solution. Please try to delete the .suo file. Or you can create a new blank solution and then add all projects with add Exiting Project way.
Best Regards,
Weiwei