I have created a custom set Part & Family's and also a DGNLIB with all the matching levels required for each family part.
But when I open the "Family/Part Editor" all the family part have "(Non Existent Level)" behand them but the levels are present in the file from the DGNLIB
Using OpenBuildings Designer Update 8
I have tried the similar steps that you have suggested.1.Created a custom level dgnlib, in the dgnlib there i have created a custom level. 2.Opened OBD and created a custom Part & family and then have assigned the custom level to the same. Till now no Non Existent Level behind the level. Tried reopening the file, still the same. Please have a look at the attached image.