The system should be smart enough to recognize feed:// or https:// 

https:// should already be allowed - are you saying you want the RSS import section to accept a URL with the feed:// protocol too?

Yes many times you want to add a feed and you go to the website and thats all you get is feed:// when you paste you get an error then have to get back to the beginning of the field to change it - if they are both valid it should just be smart to accept both protocols.