When adding a new size to an existing style you will need to upload the new sizes along with the existing size(s) in a template as normal.
Note: It is essential that the reused size be uploaded with the same information provided within core attributes (i.e. Style Name, Vendor SKU, Size, Color, etc.) it was given when originally uploaded.
Please note, if your product is 'One-Size,' you will be unable to add a new size to the existing style.
As you can see in the template below, it is best practice to ensure the core attributes match what was previously uploaded exactly to ensure properly merging and no new styles are accidently created.
After your template has successfully uploaded, you will then be able to see the new sizes merged with your existing style as shown below due. Please note that since you are altering an existing style, the product will show as 'Reused' whether the size is new or not. Furthermore, you will not be able to delete the product and thus need to discontinue it within the 'Active SKUs' catalog if necessary.