this post again.

you can't use only a 3ds you need other stuff

you need another console. if you can get a ds\dslite is better.

if you have a ds\lite you can edit the save on your pokemon black so it does have the pokemons from the other rom.

if you have another dsi\3ds you can edit the pokemon black rom and transfer it to your real black using 2 consoles.

 

no, to transfer from gen 4 you need to use a tool in gen 5 poketransfer, poketransfer will ask for a real pokemon cartridge and can't work with a rom\flashcart...

you can use the ds lite to launch the backup tool, save it on your flashcart\computer, edit the save and send the save back to your real cartridge.

you can't use a 3ds to backup your save because you can't switch cards, the 3ds will power off the cart slot once you take out the flashcart and there is no way to turn back on the power.