How do you add more shapes to the Visio Diagrams? There were some shapes I was looking for in the Sequence Diagram that aren't there. Also, are there add-ins that provide these extra shapes?
There are many Visio stencils and Add Ins available at the official website of Visio which may help you. However, if you are not able to find the required shapes, you can create your own custom shapes and add them to Visio. Refer to the link given below which guides you how to add your own shapes to Visio: