The Tekkit server package is everything you need, including the server .jar files. The Tekkit team does not include any plugins with the install. This doesn't mean that it not a Bukkit server. You can install whatever plugins you like.
As far as converting a world, it's probably possible but would be very tedious, OR you just plop it on the Tekkit server and be aware that new ores and other mod world items will only be populating in newly loaded chunks.