There are many folks on the web who successfully loaded AppKit frameworks into Catalyst apps using plugin bundles.
However, I couldn't find any information whether it's feasible in the opposite order.
I want to include an iOS framework built with Mac Catalyst in the native AppKit app. Is it possible? Any tips how this could be achieved?
This isn't possible. Bringing UIKit into an otherwise AppKit native process would break some assumptions that the Catalyst platform makes.
We refer to Catalyst as more of a platform, or technology, because there is no Catalyst framework. It's a collection of a few different things on the system that come together and make it possible. Including some classes in UIKit that only exist on the Mac. Bringing a UIView into process for example would require bringing an entire UIApplication into the process.
Some alternatives are using SwiftUI for whatever code you're trying to share, using a shared cross platform xcframework, or, if you must, creating a Catalyst extension.