Add Feeds to Mail.app from the Command Line

"Try this:

open -a Mail feed://example***/feed.xml

Firefox is probably using -[NSWorkspace openURLs:withAppBundleIdentifier:options:additionalEventParamDescriptor:launchIdentifiers:].

Another solution

You can also create RSS feeds by hand. In ~/Library/Mail/RSS/ there is an .rssmbox directory for each feed, which contains an XML property list describing the feed's settings.

The only limitation of this method is that changes won't show up in Mail until it's relaunched."