There have been a few forum threads in the past about people having trouble adding non-WordPress.com sites to their Reader and there was a staff reply given that in order to include those sites in the Reader the site must be using the JetPack plugin. How this is supposed to work with sites on tumblr, blogger, etc. I do not know.
May I suggest that you contact Support directly with your question and post the answer you receive back here in the forums. I'm sure others would be interested as well. http://en.support.wordpress.com/contact/ Support is closed until Monday.