Can someone please tell me how new tokens are added if the cryptocurrency is mintable erc20 token.
Mintable? like you can create and provide added supply to that token? There are those coins that have an unlimited supply so that's possible if the devs have decided to create a continuous 'minting' to that token that you're asking. And what do you mean by being added?
Being added to the exchanges? I'm guessing this is what you mean. If this is it then that's dependable to the devs too which coins will grant their request and how much they're going to pay to cover the listing fee.