Here is a list of ACT namespaces which you should be able to load to get added functionality to your extension:

But how do I actually add references to these namespaces into my script? I can't find anything in the documentation.

For example, going to this folder:

C:\Program Files\ANSYS Inc\v172\Addins

Gives me a lot of .dlls which I could reference to. However, Ansys.ACT.Automation.Mechanical is not listed in that folder. So how do I add a reference to it?