..none of the supplied doors appear to include a threshold...

You may also want to check the supplementary door models in the Search free 3D models page - as well as some of the user door models in the SourceForge repository .

At least 4/6 in SF 3D models 334 come with thresholds - post also shows how to use separate door and frame: to hide/display door, and to avoid that the door distorts when used in a thick or thin wall.

ok

door_frames_doors.sh3d