Creating Matrix Items from Existing Individual and Imported Items

The help documentation explains how to set up matrix items from scratch ... ie choose to create a matrix item, create the parent record, then create the matrix items.

It doesn't explain how to turn an existing item into a matrix item.  (The matrix tab is not displayed on the record of an existing item set up as an individual item.) What if the manufacturer subsequently adds other options?  How do you turn an existing single item into a parent item?  Surely it doesn't have to be set up again?

This is particularly relevant given that we cannot import our 36,000 !!!  different matrix items, we have to import our 6,000 unique items and then re-create (would you believe!) the 36,000 combinations AFTER the items have been imported into NetSuite.  But how do you do this if an existing item seems not to be able to be turned into a matrix parent item?  Is there something in the import assistant that says the imported items are to be set up as parent items?