Fill-ins are little 2 or 4 beat files a bit like each variation. Those that are part the standard rhythms built in to the keyboard can be changed by either composing replacement ones or by moving other fill-ins to replace the existing ones.
Non-standard rhythms/style files that have been imported from elsewhere usually come with their own fill-ins as well but they too can be changed as above.
As far as I am aware any settings that you organise for a rhythm apply also to the fill-ins.
Does this help or have I misunderstood you completely?