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Before finding the item, go to Products > Product Variants to create the variant

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or add the existing variant, to the appropriate Variant Group(s) for the product. Select Save.

In Product Maintenance, search for the existing matrix item that has a new variant.
Select Edit.

In the Edit General Merchandise screen of the product, in the SKU(s) section select Add Variants

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In Manage Variants, any new variants will be selectable. Choose what you would like to add and select Generate (0) SKU (number will generate based on selection).
(In image on the right, note the Yellow variant checkbox. That is the new variant that would be selected to create SKUs)

Message will display asking you to confirm or cancel the creation of new SKU(s). Select OK.

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Confirmation message will display.

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New Product Variants will appear at the bottom of the product SKU(s) list.

Make changes as needed, Select Save.