Dear Gergely,
You are correct that FAST v7 allowed active yaw control when the nacelle-yaw DOF was disabled (YawDOF = False) and that this is no longer possible in FAST v8. In FAST v7, the active yaw controller was allowed to control the nacelle-yaw angle and rate (states) directly. While this was allowed, it wasn’t recommend, because the corresponding nacelle-yaw acceleration was never calculated consistently (instead, the nacelle-yaw acceleration was assumed to be zero when YawDOF = False), so, the loads calculated by FAST may not be accurate. In FAST v8, active yaw control is only possible with the nacelle-yaw DOF enabled (YawDOF = True). While the nacelle-yaw angle and rate can be commanded, these will be passed through the second-order actuator (defined by the nacelle-yaw spring, YawSpr, and damper, YawDamp) in order to calculate the yaw moment command (YawMom).
I suggest that you set YawSpr and YawDamp appropriately to give an appropriate yaw control response. This is discussed a bit in my post dated May 11, 2010 in the following forum topic: Structural loads at pitch and yaw actuator.
Best regards,