Maybe you could try to create a new project with a simple sb3 file as a test, and see if it works so you don't waste time in case they don't. I haven't used one of those but in case it didn't work maybe you could try to compress it and send it as a downloadable file? Idk never used sb3 files, just suggesting.
So, Dannan2 said that there's this website that would allow you to change your sb3 file into an html and upload it. Maybe you can do that. When you finish, maybe you can upload it on the project and then submit your project. As I said before, maybe you could create a test project to do so and confirm if it works, but Dannan2 seems to have already given a better answer than what I could've.
