As you can see, Parity is asking you to select an Account first(which takes JSON formatted file) and use the Key Password for that account. (Please treat your MultiSig wallet as a contract because it is)
Basically, you need to have some account from which you want to execute the contract.
As I understand, you want to create a multisig wallet first(which means to deploy a contract first with its constructor parameters) and then you may want to call addOwner.
If you are trying to call
addOwner on already existing(deployed) contract, you need an address of this contract and call it using your account.
If you don't have imported account in parity, it will show you the same screen that you have right now, which is asking you to provide JSON file(account) with its corresponding password