You must have a 4.1-4.5 3DS to attempt that. You must have SaveDataFiler installed on your Emunand and CFW and the USA rom must either be converted to Card2 (if you want to run it from the red cart) or be in CIA format (if you're gonna install it on your emunand/CFW)
1-Export the save data from the donor EUR cartridge:
1.1- Set up the CFW on the 3DS.
1.2- Run the CFW, insert the EUR cart and run SaveDataFiler. Select the CTR card and export the user data to the SD card (Y).
2-Import the save data into the donee USA rom
2.1- Install the USA rom (as CIA) on a CFW or Emunand and run SaveDataFiler.. Got the 3rd tab, select your backed up data and import it to the SD card (R+A).
or
2.2- Convert the USA rom into Card2 and copy it to the red cart's microSD. Enter Gateway mode, select the USA game on the rom selection menu and run SaveDataFiler. Got the 3rd tab, select your backed up data and import it into the game (L+A)
If that doesn't work you may have to create a save file on the donee (EUR) game and export it to the SD card.
Then replace the save file inside the folder with the one from the donor rom (USA) and inject back.