There are two ways to create worlds in OpenCraft.
If you've got a world already made, just drop it into its own subdirectory under data/maps. OpenCraft supports pretty much all formats, and any unsupported format is a bug.
There is a naming convention you must follow when adding a map. Each world gets their own directory under data/maps/, and each save of a world starts with that world's name. For instance, if you have a save file named 'simcity.dat', you would copy it into opencraft as data/maps/simcity/simcity.dat. If the file was named 'simcity.mclevel', you'd save it into data/maps/simcity/simcity.mclevel.
If you're looking to create a world from scratch, you need to be in the game for that, and you need the org.opencraft.server.cmd.impl.GenerateCommand.execute permission. Once you're in, run /generate which will give you the list of parameters you need to supply.