Is it possible to create a workflow that will set the external ID of an item to be the same value as the item SKU? I'm having trouble doing this because the external ID is not on the item record itself and therefore it's not an available field I can choose in the workflow.

You can use a saved search and a import to do it. Use a case statement to show all items where the external id doesn't match the name. Would look like this: CASE WHEN {externalid} != {name} THEN 1 ELSE 0 END Show 1s

It can be done via suite script