We need a tool that allows size charts to be added to New Pre-Decorated products.

Under the "products" area in legacy, you can add the sizing charts there. That sizing chart will then show up every time you use that product. Here is a guide on adding them in the first place - https://help.inksoft.com/hc/en-us/articles/8389346189979-Add-Edit-an-Existing-Size-Chart. Once you have them there, you go to "Manage Products" and then sort by Brand, select all and then assign a sizing chart. I hope this helps.