If their sub type is assigned as a door, or relevant part of a door, they should turn up under the custom component sections of the default door objects.
ie. Assign it as a "Custom Door Leaf" and it will appear under
Door Leaf Type... = > Custom Door Leaf
Assign it as a "Custom Handle" and it will appear under
Door Leaf Type... = > Handle & Kick Board... => Handle => Custom
If it is assigned as a door itself, rather than a component of a door, it should just appear in your library under the relevant file structure that it is saved in.