Feeds that require http authentication are recently supported. Please try to use the following format, when you add your feed URL to the subscriptions:
It is important to note that such feed format will ony work with Standalone RSS service. (you can easily add it to your services with the bottom left + button)
This might not work properly for SSL feeds, but we are recently working on addressing the issue. In a future version proper management of password protected feeds will show up.
For Basecamp users this should work: