Hi,

Please refer the forum below, 

* https://www.odoo.com/sl_SI/forum/pomoc-1/unique-sku-for-each-variant-on-item-page-251869

Hope it helps

When the product template has a variants, the internal reference field will be managed by variant so you can add the SKU# for each variants in internal reference field.

From product variants menu, open the variant you want and add the SKU to internal reference.

Also you can export the product variants to excel sheet (with External ID field) and update the internal reference in excel sheet for each variants and then import it again to product variants.