I'm a Canadian Accountant with a client from the USA. Can I add them as a Client to my QuickBooks Online Accountant account?

You need to sign up a QBOA US account with another email address. 

Hello kathy-kissick-ev. Thanks for contacting us here. I'll be happy to share more info so you can get back to doing what you love. You'd need to have the US version of QuickBooks Online in order to manage a US based customer. This is under the assumption that you're using the Canadian version of QuickBooks. Feel free to ask other questions, I'll be on standby.