Get the value of custom product page type fields in Shoppingcartcontent web part transformation

Hi, I have custom page type in E-commerce site which called `Product - Shoes` and in Fields section of this page type, I've added some custom fields for example `ShoeSize` which is stored in `dbo.STORECONTENT_Shoes` table. In transformation of Shopping cart content web part I can only access default SKU fields like `<%# SKU.SKUName %>` Now how can I get value of custom fields in `dbo.STORECONTENT_Shoes` in transformation of Shoppingcartcontent web part? Thanks