To add storage to an active/active configuration configured for Multipath Storage, you need to add the new disk shelf to the end of a loop, ensuring that it is connected to the previous disk shelf and to the controller.
I haven't seen a doc saying I can hot-add a new loop.
That sounds reasonable. Adding one shelf at a time then mpha after..I prefer one change at a time like this.. I would also send an autosupport before and after the shelf addition so the configuration is sent in case of any issues. You can also open a case to review your procedure.