How can I merge existing product variants on Retail Express and Shopify?

[Shoptactical]

I'm reaching out because I am having an issue creating product variants. 

We use retail express and when we create a product on retail express it appears on Shopify if we choose, then we simply add descriptions and other details. All our inventory is stored via retail express. 

My issue is in order for inventory to be stocked I can't simply click create a variant I need to merge products which already exist on Shopify that are linked to retail express. I know it's been done before in store as an old manager did it however this detail wasn't passed on. Furthermore, I can't see any apps installed which help with this, so I'm curious as to how this has been done.

Any advice would be much appreciated.

I'm hoping this makes sense.