How to add size descriptions to shoe variants on Shopify?

[NYCK]

HI 

we selling brand name shoes on shopify, amazon, eBay, Walmart 

on many of our brands, we using something that called "size description", 

for example if we have a unisex sneaker a by Convers, the actually size would be 5 , the size description would be  5 US Men / 7  US Women, and for example VANS unisex sneaker size 5 would be 5 Men / 6.5 Women, and it would be the same with European and UK sizes , we enter all sizes tot eh system with the core size which would be 5 , however when we push it to shopify website , eBay , Walmart, we want to send it like this , 5 US Men / 7 US Women,

i do not want to enter the long description inside my POS , since it will clutter the reports , so internally i want to use the core size , however i want to push out the size description 

amazon works differently , so when we create the listing , we need to send it per amazon size specification

our current system create this in then back end, however we are looking to stop working with our current inventory management , and use shopify as our main inventory source 

i need to know if there is an app that can do this for us ? or if its possible to do this with metafiled, i know i can do it in the Metafiled variant filed , however in my theme (dawn) its only allowing me to add Product Metafiled and not variant metafiled  ,  

i need to know if there is an option to authomatcily generate these values based on condition same as we do it in collections  , and not manually