Hi Stefan Sikora,
Johannes Michalowsky suggests:
For Prosteel using DWG, in Navisworks Options set the File Format to AC 2019, then the OE will work. If using MicroStation results may differ.
HTH,Bob
Do you mind reviewing the latest recommendations (iModel and/or Beta support options) provided and/or re-asking your question (possibly in a MicroStation only context) on this thread?