I waited to see if any ES8 users would comment because I don't have a clear answer for you. The Boss ES8 manual does not state that it allows both an auxiliary switch and auxiliary button to be used in tandem, so your best option is to either connect an SP-H9 directly to your H9 or use it simply as an expression pedal. The H9 has a specific mode that allows users to use the expression pedal in conjunction with a one-button aux switch, but as far as I can tell, this mode cannot be replicated using ONE control input on the ES8.
The toe switch on the SP-H9 can be assigned to any function a normal aux switch can be assigned to. In H9 Control, under the PEDAL tab, then Aux Switch Mappings, you'll find the complete list of assignable functions.