Can p2p software automatically add files to a shared folder on its own? Has

if you have sharing turned on then yes, if you dont want to share then turn off sharing in options, kill all uploads once your done downloading a file

This is what happens with file sharing and why it is the worst possible thing to allow on any networked computer. Once you are in to this you have effectively lost control of your own machine. You can end up with something in your shared directory that could range from totally innocuous to child pornography.