I was able to add my MileagePlan number to my EI reservation over the phone. It was pretty painless once I got to an agent. However, neither segment auto-posted to my Alaska account, even though my AS MileagePlan number was clearly printed on both of my boarding passes. At 15 days past departure, I emailed a copy of my boarding pass to MileagePlan customer service and was promptly credited for my partner miles within 24 hours. (Same exact experience for both outbound and inbound flights.)