You cannot create a composite of an existing composite variant but you can create a whole new composite variant, since it is made up of several variants.
If you sell composites of your variant you might need to add a variant to you composition to create a second composition.
For example, you sell the following baby products;
You have a bundle made of all three variants, ie; Mittens and Booties . Let’s call this Baby Bundle A.
Now, in your inventory you also have Onesies which you would like to sell together with Baby Bundle A. Let's call the combination of A and the Onesie, Baby Bundle B.
However on TradeGecko you cannot add a variant to a composite variant to create another composite variant.
In order to sell Baby Bundle B, you will need to create a new, separate composite variant, comprised of Mittens, Booties and Onesies.