add anchor point to closed mask

Here's a basic tutorial I did a long time ago:

A point on a path is not an anchor point, it is a vertex. You add a vertex using the pen tool (g), modify a Bezier handle using the convert point (Alt/Option) tool, or if you are using Rotobezier paths you add a vertex and modify the Rotobezier tension or corner sharpness using the convert point tool. Switching between the select point, add point, delete point, and convert point tool is easiest if you use the Alt/Option and Ctrl/Cmnd key. Trying to edit Bezier or Rotobezier paths with the selection tool (v) can be difficult and very frustrating.

You can add points at any time with the pen tool.

Mylenium