Matt here from the Guru Team! :)
That is a great question however, Sara is correct. There currently is not a way to merge products. You can however add the separate variants by follwoing these steps here.
I hope you have an excellent day,
I found this thread searching for a solution to my own problem. I just wanted to say that one of my Shopify suppliers, Teescape, has created a tool to make it possible to combine products. Since Teescape's app is free, that means that anyone who installs their app should be able to combine products using it. A word of caution though: They clearly state that it has not been tested with products other than those created with their app. So I suggest using test products.
One limit of this app (and that's where my problem lies right now) is the fact that this can only combine variants of one products with variants of another. I have created many products with only one variant each and want to combine those... and that tool doesn't allow that.