Thanks to this forum, I managed to add my wife to my EE after accepting OINP nomination. I'm not sure how this will affect my application, but for those that might want to do the same, here are the steps in a bit more detail.
(1) login to your EE profile
(2*) check full application status
(3*) View your profile
(4*) Modify family information
(5) Click on next until you get to the part where it asks you if your spouse will accompany you. Choose yes and complete the questionnaire. (Make sure to make it all the way to the end of the questionnaire)
(6) go back to step 3 and enter the details for your spouse.
You can enter the text of the steps marked with * in your browser find/search feature. Might make finding the links easier.