Every release of Feathers UI for OpenFL so far has actually included a .swc file that can be used by ActionScript 3 compilers, including Adobe Animate. Just add it to your project's library path, like you would any other .swc file. Then, you can import feathers.controls.Button, or any other component, and use it in AS3 with nearly the same code as Haxe, in many cases.
To be clear, the Feathers UI components in the .swc may only be used in AS3 code. You won't be able to drag, position, or resize them in Animate's visual editor. It's purely code only.
Unfortunately, there are no samples that demonstrate how to use Feathers UI for OpenFL in AS3. As I said, the code is very similar, and hopefully, most developers can figure out what to tweak for AS3 based on the Haxe examples. My main focus for these components is on Haxe, and it was relatively easy to build a .swc, so I figured that I would provide it, and maybe some folks would find it useful.