Can BOM routings be re-imported?

You cannot add routings (using the import) to an existing BOM revision.  The BOM Routings import creates a new BOM revision for the BOM in question.  So, you must make sure the revision you're adding doesn't already exist under that BOM parent.  If it does, you'll need to either delete it first or you'll need to import under a new revision.

After you've used the routings import to set up a new revision for the BOM parent, you can then use the BOM Components import to add components (and in that you can link them to the routing sequences you have imported).