Yes. You are welcome to offer your own program (or partner with a third-party rewards provider) to give cash back or other rewards. Every quarter, you will receive a percentage of all interchange revenue that Hydrogen gets from the payment processors. You are welcome to use this revenue to fund your rewards program, or use money from other sources.
Please make sure you follow the marketing and compliance guidelines located in your Hydrogen admin portal. If you are unsure of the legal or accounting implications of offering any rewards program, please contact a lawyer or CPA.