A simple way, probably not. The base in NNF does not offer permanent logins (I am never logged in, always enter my password every time I post), and by default if someone can guess the name of the RSS feed everything is publicly readable as well.
I would personally just not recommend sending private message, if I know they are all stored in an RSS feed that is readable by anyone with the link. There are better platforms for messaging.
If you have to figure out a different storage mechanism for messages than posts, you have long left the simply way behind.