Kallie You could technically "hijack" the event handlers on the "Add attachment" button and do your own thing. There are many ways of doing something like that:
a) the JavaScript File API could be used to send the selected file to your server
b) FreeMarker also offers some options with Lithiums own http.request context object
http.request.uploads.name.get("<file_input_name>")then send the file to your server with a post request somehow...
of course you need to handle the reverse as well, e.g., you need to figure out a way to tell your server which file belongs to which post, then when a user is on post you need to query your server for attachments and so on... why would you want to do that? (just curious)
