At this time, only individuals with Classmates.com accounts will show in the drop-down list of possible names, and on the Memorial Wall list. But you can post and share memories about anybody from your class (whether they were a member of Classmates or not) under the Share your memories section of the page, or even post a list of names there.
To do so, please go to your class Memorials page at http://www.classmates.com/siteui/memorials/ and type your message under the Share your memories section. Please be sure to include the full name of your schoolmate(s) in your post.
Your post will show for any class members who visit the page, and they can share their own memories in the same section.