Once you retire, are you able to add an award to your DD 214 if you receive your retirement award late??

Yes. You have to file a DD215 to have your 214 amended.

Yes but you need verification

I got out in Nov 1975 with 10 1/2 years service. Was told by 1SGT that I had been recommended for the Meritorious Service Medal. Never received it or heard anything about it until 2006 when I wrote the MilPerCen for a list of awards I was entitled to. Number one on the list was the MSM with and award date 29Dec75. Took me 31 years to learn of the award. And the interesting part was I have made a similar request about 5 years earlier and the award did not show up. Included with the packet was a copy of the orders awarding the medal, but no certificate. In 2014 I again contacted them for a copy of all my military records and lo and behold, there was a copy of the award certificate. Not the original, but a barely legible xerox copy. I received a DD-215 showing the award.