First I would recommend reading through this
https://docs.spongepowered.org/en/server/getting-started/migrating.html
Then I would check out Project Worlds. Importing bukkit worlds is still a WIP so results may vary
I do beleive EssentialsCmd also has some form of World management as well but personally haven’t checked it out.
I would also browse the forum, I believe there is a few posts floating around regarding importing bukkit worlds