Yes you can.my N3DS XL is already hacked and I have a cartridge copy of Ocarina of Time. Can I hack my girlfriends N3DS XL with that?
Use the OOT3Dhax installer to enter the FW of your girlfriends 3ds, then put the cart in her 3ds and load the save, and teh HBL will load.
Don't forget to put the hombrew starter pack on her 3ds SD Card.