Yes you can. You can download the plugin API documentation at this page:
http://www.msgplus.net/help_plugins.php .
However, Messenger Plus! Live will be released soon and that version won't have support for plugins anymore. It will only have support for scripting which works in a different way. That means that any plugin you create now won't work in any next version of Messenger Plus!. I suggest you wait with creating you plugin and wait for the next version and scripting documentation.