Interestingly enough - yes you can! :-) I've done this a while ago and it is not that hard!
So, what you need to do is to use your FTP program (I use FileZilla) and go to the wp-includes folder in your root WordPress directory. After that find the file named default-widgets.php. Open it with NotePad ++ (or another text editor) and find this class: a class='rsswidget' (I believe it was on line 783) - after that it is easy - simple add the attributes you need such as rel="nofollow" and target="_blank". Now your RSS feeds are tuned up!
This is for the main title. If you want the extracted content to be nofollow as well then search for 'rsswidget' again! find the <a href> and replace what is needed! Also note! Whenever there is WordPress update the file will be replaced and you will need to the this job again! So better keep a backup of your modified file!
EASY! If you know where to search! :-) Cheers, Alex