This feature is only available on our Advanced Plan
On the Dashboard, click the edit size chart button for the size chart you want to add to another size chart.
Scroll down to the bottom of the page.
Where it says Parent Size Chart click the dropdown and find the size chart you want to add the current size chart to.
Make sure the Parent Size Chart is higher on the list of size chart in the dashboard than the current size chart or the current size chart will override the parent.