>Is there any way to index (or reindex) tables which are not opened exclusively?

It depends on what kind of indexing you mean. Adding structural index tags or reindexing surely needs exclusive open, but creating other indexes can be done even on read-only tables - anything except creating/deleting tags in the .cdx which has the same filename as the table. This means you can create temporary indexes anytime you like, provided they don't collide with any other user using these .cdx or .idx files at the same time.