You can create a parent product and have the variants be managed by the Turn 14 app. The key here is to make sure you use the exact same sku as the initial product loaded by the app. This is case sensitive, so make sure to use the same format! You also need to make sure the variants have the Turn 14 app specified in the "Inventory managed by" setting in the product page.
Any product in your store can be a parent product. You just scroll down on the product page:
When you add a variant you can then edit it:
Then switch the sku to the turn 14 sku you want to make into a child/variant product:
