does anybody knows where i can get a underslung rear brake adapter for the 990 superduke ?
want to mount brembo cnc 32 with 84mm

No, but the two piston from adventure fits and can be matched with appropriately sized master. Not the best but still better than the feeble stock setup.

If you are experiencing rear brake fade, why not just get a brake line cooler from Slavens Racing. You don't really need more rear braking, you just don't want it to stop working when you need it. A cooler will help keep brake fade to a minimum if you are using it a lot.

Otherwise, like Willh said, the twin pad rear caliper from the 990 adventure, SMR or SMT will slide right on the SD. You will still have the same issue with brake fade though once the fluid gets hot.

i thought that there is a plug and play solution , but ok , then i have to use the adventure mod