For an existing Product, you should have Style Code ready. And make sure the Size Curve you used for this product has the SIZE you want to create VariantSKU (Stockcode) for.
Go to menu Product -> Create (Auto) screen (see below), in the Style Code box, delete NEW and start keying in the StyleCode for example NNN0055. As you are keying in, you should be able to see auto-fill helping you.
Now, select the style and press <return> key. All the details of that style will be filled in to the text boxes.
When done, in the Size Curve grid,
> scroll down to the size to you want to create the variant SKU for
> check the box
> input cost, retail etc. and click SAVE button. (please see screenshot below)
When done, you should see the new Variant SKU (Stockcode) in the system for chosen Size.
If you need further help with this process, please email us at supportdesk@modulusretail.com