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The biggest problem when dealing with this is the complexity of the project. Importing DWG files is time consuming when you have to break everything out. DGN files take even longer. The platforms don't mix well. If at all possible I would suggest using thier documents to recreate the essential architectural elements into the REVIT model and then "hide" those added elements prior to exporting for the Microstation companies use.
Has anyone looked into using and iModel from Revit? Bentley base their whole interchange between their suites on the iModel formats. They also provide the plugin to create them directly from Revit.