To export ERC20 tokens from Atomic Wallet you need to Export your Ethereum private key.
The reason of it is that every token is attached to your Ethereum address. They are all running on Ethereum Smart Contracts technology.
And the management of this tokens requires a small amount of Ethereum on your address.
If you need to know how to export private keys from Atomic Wallet -> Read this aticle.
In the meantime, check out our XRP Wallet.