It is now possible to select a 'National ID' as a Crew 'TRAVEL DOCUMENT' on a flight.
In order to select the 'National ID', it needs to be added to the User profile.
First step is editing CREW profile and clicking a button
Second step is adding details, as displayed on the screenshot:
Once the 'National ID' is added it can be selected on a flight in the 'CREW' tab > 'TRAVEL DOCUMENT' section.