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This question already has an answer here:

How can you add airdrop functionality to an already deployed smart contract on the mist wallet?

marked as duplicate by Richard Horrocks, Thomas Clowes, Malone, Jesse Busman, flygoing Dec 28 '17 at 2:07

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You don't actually need to add the Airdrop functionality to an existing smart contract - which is not possible-, you could just create a separate smart contract that interacts with your ERC20 token.

You'd just need to transfer the tokens from your balance in the ERC20 token contract to this new contract which would have a function that distributes some tokens to a list of addresses.

Here's an article I recently wrote, which could give you a few pointers on how to build such a contract: https://medium.freecodecamp.org/how-i-built-a-multi-token-airdrop-central-to-distribute-erc20-tokens-cb70b6218b5c

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