Good question. Unfortunately, we have bad news: SSH agent support won't come. Why is that?
Apple requires all apps submitted to the Mac App Store to be sandboxed. That means, an app has only access to generally shared and explicitly selected files of a user - for example, your SSH key pair, but only after you selected it in the Dropshare for Mac preferences. However, within the Sandbox, Dropshare for Mac is not able (as in not allowed) to communicate with the OS X SSH agent.
You can read a very good and in-depth explanation in the Atlassian Blog, as they had the same problem with their Git client SourceTree.