I bought my wife's Moto X Pure Edition from Best Buy and added Moto Care. Before I purchased from Best Buy, I confirmed I could add it within 30 days if the device was bought elsewhere (Amazon or BestBuy). I was assured I could.
When I went to add it (by phone I think), the first person I spoke to said I couldn't add it if Verizon was my carrier. I was a little steamed, but ended up calling again and speaking to someone else and was able to add it. IIRC, it was an odd process where I logged into my motorola account and they made it available for me to select (i.e. since I already had the device).
The only glitch was the Effective Date was ~1 month before I got the phone. After speaking to a few people, no one could understand why that was. I ordered the phone from BestBuy on 10/14 and received/activated it on 10/21, yet my MotoCare contract says effective date is 9/16/2015 - 9/17/2017. Eventually the rep just told me if I have a claim between 9/17/2017 and 10/21/2017 and can prove I didn't receive the phone until 10/21, they will honor it. I wondered if the 9/16 date was the date that IMEI was shipped to BestBuy.
This process is in contrast to Nexus Care, where you can *only* get Nexus Care when you order the device through the google store. I just bought a 5X for my son (with Nexus Care) and ordered through the google store rather than Amazon.