How to install flat mount road disc brake calipers on IS mount bike?
As of now there is no commercial product you can buy to make this possible. An adapter that did this would be awkward to design because the flat mount caliper, in its native spatial position relative to the rotor and stay/blade, is trying to occupy some of the same space as the IS tabs. I believe it's physically possible but it would have to resort to being bulky and positioning the caliper quite far away along the rotor from where the tabs sit, which is a very negative design trade-off. It may even raise questions about how much leverage the tabs or mounting bolts can handle.