I don't think you can do it this way in the renderer.
What do you mean by "extending" and existing activity module? Are you cloning it or trying to shoe-horn addition bits in.
Depending on the module you may have some luck if there are existing hooks.
For example you can (with a bit of jiggery pokery) use submission and feedback plugins to modify the assignment activities edit form, but only the bits *above* the submission and feedback plugins section.