Yes, sure, in accordance with TGL-10, when you're flying PRNAV procedure you are not allowed to modify or insert waypoints. That's true. That's why, I think, when I choose DIR function, my FMC (neither Collins nor Honeywell) actually flew over the waypoint, hardcoded as 'Fly-over'. It's flying only as hardcoded, without modifying 'fly-over' to 'fly-by' in DIR mode.
In my case, it was unexpected for me and ATC, because after DIR my plane overflew the waypoint and thereby overshoot the track.