rferdinand,
I have not seen any actual reference that shows this. Making a new bow and shaping an existing one are not quite the same thing, but the tools and methods would be similar. Possibly look in some books on bow building, or you could just find a bow that has a grip similar to one you want(One that fits your hand better) and just start working your Sage down with tools until you get to where you want to go.
The up side to this is that the Sage is not a collector’s piece and new/used duplicate risers are available if you go too far.