Q:

Creating a custom SSRS control

Is it possible to create a custom third party for Reporting Services? 
The Dundas chart controls are an example of this however I am unsure if support for these have been internally built into SSRS..

A:

Yes.
After consulting google I've found that SSRS has CRIs (Custom Report Items). These can be created in a .NET language by implementing the ICustomReportItem interface (for the rendering) and inheriting from the CustomReportItemDesigner class (for the designer). These use the Microsoft.ReportDesigner and Microsoft.ReportingServices.Interfaces namespaces. 
There is also a restriction in CRIs that the end output needs to be an image. This would make any interactivity very difficult.
For more information see
Jazz Up Your Data Using Custom Report Items In SQL Server Reporting Services
Creating a Custom Report Item

