As noted in linked question all addresses exist.
If you want to know if for a given address a private key exists (e.g. this address can send transactions) you can have the method
register() which is supposed to be called by normal users. This method will store
msg.sender to the
approve(address addr) is supposed to be called by the super user and it will put the address to
approvedAddresses mapping, after checking the
addr exists in the
registeredAddresses mapping. The fact that this address is in
registeredAddresses means that someone must own the private key for this address.
This of course imposes the gas cost that users will need to pay for calling
register(). Instead you can also do it off-chain, just ask the address that you need to add to the database to sign some random message with their private key then check it against the address with