I would like to add a dynamixel submerged motor to my BlueRov r3 and control it in position (In the same manner as the servo-motor of the camera), How I can do this ?
Hi @saddam, welcome to the forum 
From a quick search it looks like there are setup instructions for Dynamixel motors in the general ArduPilot documentation, which means there’s an ArduPilot driver for them that is likely accessible to ArduSub as well.
That said, it’s not something we’ve done before, so for help with how to actually control them your best bet is likely asking on the general ArduPilot forums. This thread may be a good starting point.