Currently its not possible to see the properties added to the imodel (inherited from the ItemTypes in the original DGN), in the IFC model after export. Further it would be relevant to map the properties to various classification systems like UniClass, CoClass, CCS, etc., according to client requirement.
Could that mapping be a part of the JSON file during the IFC export?
In separate emails Mikkel said that this was actually solved by getting the data into the source dgn and this was no longer an issue for the IFC Export