Can you use to adhere a expanded polystyrene shower bench to hardi backerboard or would modified thinset mortar be the preferred application?

Thank you for the question. There is not enough details about the bench to provide an adequate answer. For example, is this a free-standing bench (no bottom support) or does it have a base support. It sounds like you need to consult the bench manufacturer for proper attachment instructions. Using an adhesive will also depending upon the available bonding surface you have. Again, this information is not provided.