Solution
Yes, DIAdem can be used with CAN FD.
The DIAdem NI-XNET driver uses the NI-XNET driver for the communication with the CAN card. Therefore, CAN-FD cards and CAN FD Frames will work with DIAdem, as long as they are supported by the NI-XNET driver.
When using CAN FD with DIAdem DAC, for the NI-XNET Signal Selection, in the Bus Configuration, select the High Speed (HS) Tranceiver option. As CAN FD is the predecessor to High Speed CAN.