This is already possible without custom fields using a normal post, albeit perhaps with a little more manual work than what WP can offer with a plugin.
- Create new post and enter the title as the same as the article you’re linking to
- Enter the excerpt of the article you’re linking to as a quotation in the post body, plus any other commentary you’d like to add
- The except can normally be found by opening up developer tools in your browser, expanding the
<HEAD> section and looking for the
<meta property="og:description" content="My amazing description about the best article I've ever written"> tag.
- Go to Post settings > Meta data and paste the URL of the article you’re linking to into the canonical URL field
- Hit post
I don’t think Ghost is trying to be anything like a CMS; it’s specifically designed for publishing rather than acting as an all-in-one CMS and website builder.
Post sharing links are part of your theme, so you can already do this without custom fields by editing your theme. Bear in mind that most people tend to use mobile devices these days, so even if your theme doesn’t have any sharing options built in, every mobile browser does through the iOS/Android Share Sheet functionality.
Those share buttons do add quite a bit of bloat and visitor tracking though, so there are some trade-offs to be made.
What are you trying to achieve? What sort of feature are you trying to build into your website? How are the current suite of Ghost features and tools coming up short? What sort of roadblocks are you hitting?
There’s lots of different ways to achieve the same goal. Custom post types in WordPress added almost limitless customisation options, but that doesn’t mean it was a good idea. That limitless also added significant complexity, bloat, and theme customisation to make it work properly.