First off, your 3DS needs to be hacked, sorry if that part is obvious, but there's no way to share save files without having a hacked system.
After that, there are a lot of different apps that back up and restore saves, but Checkpoint, which you can download here
is the most current one and the one I use. It's a homebrew that you install on a hacked 3DS (either as a CIA or a 3dsx for the Homebrew Launcher). When you run it, it will load all of your games and if you select one, you can choose to back up or restore saves for that game. The way it works is that every game gets its own backup folder on your 3DS SD card, and the folder for your version of Lego Avengers is where you'd need to copy my save if you want to try importing it.
There's actually lots of separate European releases of the game (France, Italy, etc.), so I can't say for certain which version you received in Greece, but if you have the generic "Europe" version, the folder you're looking for will probably be named something very close to 0x01680 LEGO® MARVEL's Avengers
. It might not be exactly that, but that's the basic idea. The name could be different in a different region, and the number at the start is always different for each version, e.g. the USA one is 0x01685
, Europe is 0x01680
, France is 0x01681
Once a save is in the folder for the game you want to import it to, you can go back into Checkpoint and select the game, and you should now see the save in the list in the bottom left, at which point you can choose to restore the save and it will be copied to your game.
Then you run the game and hope that Lego Avengers saves are compatible across regions.