To answer your questions, yes, you would submit a Learning Plan to extend your practice of clinical psychology to include couples. The process is the same for a psychologist and a candidate registrant. For more information you can see page 21 of the Supervision Handbook:
https://www.ns-rp.ca/downloads/SupervisionHandbook.pdf nThe Learning Plan can be submitted for consideration at the same meeting as your CFTA Transfer application, which will be the next NSRP Meeting once all application requirements have been met.
Alternatively, you can request an amendment to your scope of practice from the College of Psychologists of Ontario. Should they amend your authorized scope of practice to include couples then NSRP would accept this change without the need for a Learning Plan.