oh, newsblur IS my rss reader! I am a paid user but they have a free tier as well (just fewer feeds allowed iirc).

the way you add newsletters (or any emails tbh) is you go to your email service and set up a filtering rule for the kinds of emails you want to put into your newsblur rss reader, and you set that rule to forward all those emails to a specific email address that newsblur provides you with. and all the emails that get forwarded to that special email address show up in your newsblur just like any rss would! it's very cool.