How can I move my character including all its content from one world (save) to another world (save)?

Since I'm playing with about 100+ mods and in singleplayer survival, I want to simulate Multiverse from Bukkit in my singleplayer mode this way.

I want to create multiple saves and only transfer the character and its current content to the new world/save, and after a while back to my original world.

Is that even possible?

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!

The easiest way I've found to transfer your player to another world involves using iChun's Sync mod and MCedit.

First of all back up your worlds. You should always do this before tinkering with the game's files in any substantial way, anyway.

In the world are transferring from, make a clone with a Shell Constructor and pig(s) on treadmill(s), make a Shell Storage (don't forget to activate it with the lever) and use it to switch from your old player (henceforth we'll call him transferPlayer) to your new clone. Pause the game and click "Save and Quit to Title".

Open the world you were just playing in MCedit and then find the Shell Storage with transferPlayer in it and export it as a schematic (include a few extra layers of blocks on the side the lever is on, as the lever must be included with the shell storage).

Open the target world to which you want to transfer transferPlayer in MCEdit and place the Shell storage containing him. Save and close the world. Exit MCEdit

In the world where you placed shell storage in the previous step, create another Shell storage and switch from the old player from that world to transferPlayer. With that, your player should have been successfully transferred.

Lastly, turn off the power to kill the original player from this world. This way, when you die, you respawn and don't go back to original player for this world.

Done with MCEdit-0.1.7.1 and Sync 2.2.3.