Hey sorry for the delay. You don't specifically need spritesheets to import the animations, Just dragging in the keyframes is enough, Unity can build them from those. There's also a second way, using the Spriter importer, That way you don't have to import hundreds of images and you get smoother animations, it's a bit more complex and sometimes can be buggy. Here's the link. If you have any issues drop onto my site's discord server there are quite a few unity dev's there that have experience importing my sprites into unity with the Spriter importer.
https://github.com/TerminalJack/Spriter2UnityDX