You cannot create a variant of an existing Bundle but you can create a whole new Bundle since it is made up of several variants.
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 bundle to create another bundle.
In order to sell Baby Bundle B, you will need to create a new, separate bundle, comprised of Mittens, Booties and Onesies.