Withered flowers can be added to Bokashi Organko without any worry. Just make sure they are not moldy and cut them into smaller pieces.
You can add small amounts of dead flowers and stems into the bokashi bin. The only thing you need to keep an eye on is that there is no mold present on them as in excessive amounts, this could affect the fermentation process. Make sure to cut them into small pieces and follow the instructions for bokashi composting.