Thanks for joining us and congrats on getting back into this hobby! I shared a video about converting the SK8CAD STLs to STEPs with free software on my Patreon page a little while back: https://www.patreon.com/posts/convert-stl-to-67586989
It's not specifically about Fusion 360, but I expect it will work: a STEP file is a solid model, and you can import the STEP files you create with this tutorial into Fusion 360. Let us know if this works for you!