My company (Tidings) does just this concept, although we don't currently integrate with MailUp :/ You can add your feed, drag the articles you want, and then copy and paste the HTML of each email into MailUp, but I realize that last step is less than ideal. It's free trial if you want to give that process a shot, though.
We have a number of customers combining feeds and emails. Here are a couple ideas that don't use our service--
Zapier has a ton of possibilities around feeds and emails. It looks like Mailup's Zapier integration doesn't allow you to generate email content, but perhaps you could email the MailUp team and get them to add it to the product roadmap.
Another idea, if you’re just trying to save individual articles to a given feed, the best way would be to create a Pocket account and install the browser extension. Then save articles to Pocket as you come across them. Then create a feed for your saved articles as described here: https://help.getpocket.com/article/1074-can-i-subscribe-to-my-list-via-rss
Hope one of those is helpful!