How do I add an existing shared data source and dataset to my SSDT/Visual Studio SSRS Project?
Here is an explanation retrieved from various MSDN and TechNet articles.

Prerequisites are


  To create a shared data source or modify its properties, you must have Manage data sources permissions on the report server. If the report server runs in native mode, you can use Report Manager to create and configure the shared data source. If the report server runs in SharePoint integrated mode, you can use the application pages on a SharePoint site. For any report server regardless of its mode, you can create a shared data source in Report Designer and then publish it to a target server.


To import an existing data source in Report Designer


In Solution Explorer, right-click the Shared Data Sources folder in the report server project, and then click Add Existing Item. The Add Existing Item dialog box opens.  
Navigate to an existing Report Definition Shared data source (rds) file and then click Open.  
Click OK.  


Adding Report Data

In Report Builder or Report Designer, you can add data in the following ways.


Add embedded datasets based on shared data sources.
Add embedded datasets based on embedded data sources.


A detailed explanation can be found here: Report Embedded Datasets and Shared Datasets (Report Builder and SSRS)

There are some limitation in Report Designer:


  In Report Designer, you can create shared datasets as part of a report project, and control whether to deploy them to a report server. You cannot browse to a report server and select a shared dataset to add to your report.


List of references:


Create, Modify, and Delete Shared Data Sources (SSRS)  
Add and Verify a Data Connection (Report Builder and SSRS)  
Report Datasets (SSRS)  
Report Embedded Datasets and Shared Datasets (Report Builder and SSRS)