I'm thinking you'd like your brake light power to also light up your rear directional signals?
The factory wiring doesn't allow for the above without wiring modifications made both upstream & downstream of the signal light switch on the handle bars.
Then there is the issue of splitting out the brake light power from the front directional lamps, and also how to make the directional light signal take priority over the brake light signal.
Not impossible, but it will take some time to do.
If you can find a schematic for a Signal Stat 900 controller, it should show how this can be accomplished.