Yes, you can use an existing Macquarie CMA into Stake.
You will need to complete only sections 1 & sign section 6 of the attached Third Party Authority Form, which grants us general authority over the account. This allows Stake to arrange the transfers from your CMA to your US trading account. It also means you can see the balance of that account in Stake.
Once complete, please scan and send back to us at firstname.lastname@example.org