Yes you can. The easiest way would be to use World Edit
(
http://dev.bukkit.org/server-mods/worldedit/)
You would have to run a localhost server (A server only you can get into; make sure it has Bukkit and you have World Edit loaded on the server properly.)
You can either world hop (by changing the names of the world back and forth), or you can add another plugin that will allow you to teleport to each world more easily.
Make sure you have all of you plugins correctly installed. (for this part you must know how to use the
//copy //paste commands for world edit) Copy the region you want from one world and paste it into the other. Keep repeating this step until you are satisfied with your world. If your world looks choppy and unnatural, then you can use the
/brush smooth tool (world edit).
~Craftee
