MultiSigWallet | Allows multiple parties to agree on transactions | Cryptocurrency library

Adding and removing an owner follows the same rules as confirming any transactions from the MultiSigWallet. Assume the wallet was deployed with 3 EOA addresses as owners and requiring 2 confirmations for execution. For any regular transaction, say donate funds from the wallet to a FundRaisingContract, one of the owners would need to first call submitTransaction() passing in the address of FundRaisingContract, the amount of ether, and the hash for the donate function. After submission, you still need 1 of the other owners to confirmTransaction (the submitter is automatically confirmed). Once completed, now executeTransaction can be successfully run which will execute the call to donate() from the MultiSigWallet address.

That last part is what you're looking for to answer your question. To add an owner, repeat the example but use the MultiSigWallet address and the hash of the addOwner method. When one of the other owners confirms the transaction, addOwner will be called and it will pass the onlyWallet modifier check.