Can I put a lock on an already issued erc20 token? I have issued a fixed erc20 and I want to know if it is possible to apply locku to smart contracts.
The token supports only the functionality written into the token's smart contract. I'm not sure what you mean with "lock" but most likely that is not possible unless you have originally coded such functionality inside the contract.
If it's a basic ERC20 contract with no extra functionality then there's very little you can do about the tokens. Depends a bit on your initial release method (ICO or something else?).