Yes, but not easily.
I'll assume your Minecraft name is "NoCanDo" for convenience. You can copy the NoCanDo.dat file from the players folder inside a save to another save. This file stores all your inventory, experience, etc. You also need to copy all of the 'Level.dat' files, found in the world save.
That's the easy part. The problem is that it also stores your location and respawn (bed) location, which may be underground or a hundred blocks in the air in the world you just copied it into, and will likely result in immediate death if it's just copied straight across.
You can fix that by hand, and this is where the "not easily" part comes in. Using a tool like NBTExplorer you can open a save and view the contents of a player's .dat file and modify the data directly. Once you've copied the .dat over, open the destination save in NBTExplorer and navigate to [savename]/players/NoCanDo.dat/Pos and set the three numbers under Pos to a safe location in your new world. I ususally choose the spawn point, since that's reliably safe and I can find the exact coordinates with NBTExplorer inside the level.dat of the save. The numbers under Pos aren't labeled, so remember that they go in X, Y, Z order from top to bottom.
You can similarly change your respawn location by editing the SpawnX, SpawnY, and SpawnZ entries in a players .dat file, or you can skip that fiddling and just sleep in a bed as soon as you can.
As always, make backups!
There is a simple way to combine your Multiplayer world for Single-player use.
Notice how your single-player worlds are saved in region, DIM1 and DIM-1 folders. Each of these folders is your dimension.
Multiplayer servers saves each dimension in its own folder, while the game (single-player) saves it as a whole folder of its own.
So, what you want to do is copy the DIM1 (Nether) and DIM-1 (The End) into the folder with the overworld dimension in it (level folder; as it is called in your screenshot), and then you can put your level world folder straight into your saves folder, in the Users/[Username]/Library/Application Support/minecraft/ folder, which then will run perfectly fine with all your dimensions intact.
Player inventories, locations etc. are stored in the overworld folder (level), so all locations will be preserved. (Means if you were in the nether, you'll log back on into the nether, or where the level.dat says you are)
You should end up with a file directory like such:
...
- local
- ...
- minefold
- ...
- level
- data
- ...
- DIM1
- ...
- DIM-1
- ...
- players
- ...
- region
- ...
- uid.dat
- level.dat
- level.dat_old
- session.lock
Note: "..." means things not included/not important.
The DIM1 folder contains data for your Nether,
The DIM-1 folder contains data for The End and
The region folder contains data for your overworld.
Everything else, you do not need to touch.
Best Answer
The world data, which also includes player inventories, is saved to a folder in the current working directory of the server.
The name of said folder depends on the
level-namesetting inserver.properties, and defaults toworld. Thus, you can start a new world and toggle between the saves by simply changing this setting.Vanilla server commands related to saving/backups:
/save-all- forces the server to save./save-off- disables saving. You should definitely execute this before making a backup of a running server to ensure that the server doesn't save while you're copying, which could result in a corrupt backup./save-on- re-enables saving.