I don't think Nikon makes an extension for the SC-28. You could set the 910 to SU-4 mode where it is in optical slave mode, and trigger it with your on camera flash. You'd have to set the flash up to either auto or manual 1/1. Otherwise you need the SU-800 to utilize remote CLS functions, either TTL or manual, but controllable from the Commander. That way you avoid any interference from the onboard flash.
EDIT: You can play with manual settings. M 1/1 is just the starting point. You can dial down to 1/128.