I have opened several DGN that have been linked to a Workset. When I try and add them to the sheet index. All of them report and error stating they are not linked to a workset.
Yet the properties say they are.
WHY?
There should be a WorkSpace field under the WorkSet properties.
Can you follow the instructions in the following wiki to disassociate the file:
https://communities.bentley.com/products/microstation/w/microstation__wiki/32023/how-to-break-the-association-between-certain-files-and-a-specific-workset
You should then be able to reassociate to the WorkSpace and WorkSet.