You can't trade, but you can transfer from HeartGold/SoulSilver to Black/White/Black 2/White 2. Though they can't be sent back once transferred.
Requirements
- Two Nintendo DS/3DS/2DS systems.
- Legitimate copies of Pokémon HG/SS and BW/B2W2 (if you have a bootleg game, you might not be able to transfer).
- In HG/SS, you need to defeat the Elite Four and enter the Hall of Fame, then try to board the S.S. Aqua to get the National Pokédex.
- In BW/B2W2, you need to defeat the Elite Four and enter the Hall of Fame, then go to the PokéTransfer Lab on Route 15.
How to transfer
To transfer Pokémon from Pokémon HG/SS to Pokémon BW/B2W2, go to Route 15 in BW/B2W2 and go into the PokéTransfer Lab. Go inside. If this is your first visit, a scientist will talk to you and take you to the top of the room. If you have visited before, go talk to the scientist at the top.
When asked to use another DS, choose Yes. When you are informed that the Pokémon can't be sent back, choose Yes. When prompted to start DS Wireless Communications, choose Yes. When asked to save the game, choose Yes.
Turn on your other Nintendo DS/3DS/2DS that has the Pokémon HG/SS cartridge in it, then choose DS Download Play from the menu (if using a 3DS, Download Play is in the home screen. Look for the orange icon with a DS on it. When prompted, choose to download Nintendo DS software). Select the game that has the ID number that is displayed in your Pokémon Black 2 game, then say Yes to download. After it downloads, choose exactly six pokémon that you want to transfer, and say Yes when asked if you want to transfer those pokémon. Say Yes if the game tells you that held items will be put into the bag.
After you have chosen the Pokémon, you will play a mini-game in Pokémon Black 2 where you throw Pokéballs at pokémon that move around the screen. To throw the Pokéballs, drag the stylus on the touchscreen to pull the Pokéball downward, and watch the top screen to see where the Pokéball will land. Take the stylus off of the screen to throw the pokéball. If a pokémon is hiding in a bush, throw a Pokéball at the bush to make the pokémon jump out. Throw a Pokéball at a jumping pokémon to catch it. Near the end of the time limit, a pink cloud of Sleep Powder will float across the screen. Hit the pink cloud with a Pokéball to make the pokémon fall asleep. Then throw Pokéballs at the sleeping pokémon. If a pokémon is not on the screen when you hit the pink cloud, you won't be able to catch that pokémon until it wakes up and comes back onscreen.
If there are any pokémon that you do not want to send from Pokémon HG/SS to Pokémon BW/B2W2, just don't catch those pokémon. Uncaught pokémon will go back to Pokémon HG/SS.
After the time runs out, you will be asked if you want to transfer the caught pokémon. If you say No, the pokémon will go back to Pokémon HG/SS. Otherwise, say Yes to transfer them. The game will be saved. You will be asked if you want to use the PokéTransfer again. Choose No if you are done, or choose Yes if you want to transfer more pokémon. If you are done, the game will ask you to turn off the power.
In Pokémon BW/B2W2, the caught pokémon will be sent to your PC Box.
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