World Download

I'm trying to download the world of a multiplayer survival server, since I want to be able to work out a build a bit in a creative world. However, when I download the world, I'm locked in survival mode and can't run any commands. Is there a way to download only the world without the player data or edit it once it's downloaded so I can run cheats in the downloaded version? I can't just use the world seed, since we've already built some things and I want to see how a build would work with existing stuff.

I tried that, and then went to multiplayer, typed in the IP, and it says it's not available. Do I need to use multiple accounts for this to work?

I couldn't figure the LAN out, but I got NBTexplorer and messed with the tags until it worked lol