Yes, you can add someone who is not a Flickr member to a photo. People in photos is all about featuring Flickr's current members. If you want to add someone who is not a Flickr member (non-Flickr member), then you can add them to the photo using their email address. Other members will not be able to see it. Flickr will send an email to those non-Flickr members to introduce the Flickr and the People in Photos feature.
You can install PicBackMan's Flickr uploader for Mac from the website and start backing up photos and videos.