No - you can only use cash. Obviously this is because Octopus cards are fully refundable in cash so there is an obvious mileage scam if you allow card purchases.

The only exception to this is that you can link your Octopus card to be auto-recharged from your (HK-based) credit card when it reaches a certain low balance. That way you can earn miles on your Octopus spending, but I am not aware of any way to get cash back from an Octopus card without handing it back.

Although it is easy to see how to get a few miles for free, I don't think this scales well - i.e. it would be a lot of hassle, and almost certainly "trapped" if you do it repeatedly.