Maybe some cc messages can be added inside the style's track within the instrument or other pc tools?
Could be a work around..
But if this is possible, then it is offcourse not a hardware limitation, as AJ indicates, but a software limitation..
Also i been looking for a midi cc implementation list of the sd90, but is not easy to find, so far it seems not out there, all i have foun courtesy of Kurt is an sd40 sysex list..