I purchased a standard Schluter®-KERDI-DRAIN bonding flange with a 2" outlet; I now need the adaptor ring for an existing clamping ring drain. Will I be able to use this KERDI-DRAIN flange with the adaptor ring?

No, the KERDI-DRAIN bonding flange is not compatible with the KERDI-DRAIN adaptor ring. You will need to use a KERDI-DRAIN adaptor kit in this case, which includes both the adaptor ring and bonding flange.