If you're referring to assigning a foreign currency to a customer, yes you can use the USD currency as a functional one in QuickBooks Online Hongkong region. You'll need to create a new customer profile and choose the foreign currency that they use to pay you for products or services. Then, create the transaction and assign the USD currency to your customer.
One problem that you might run into is that QBO reporting for multi currency is pretty limited. There is no way to report on the foreign balance of an account that has a currency different than your home currency. The only way to report the value is in the home currency but this is often not what is needed.