Can you clarify a little more? Are you trying to...
- Add a newly created Variant to an existing product dynamically.
- Add an existing Variant to the cart
- Add a category object to your product
It sounds like #3 is what you are trying to do, in which case it should help to know that an added product category has it's own SKUID, and when added to the cart it should set the ShoppingCartSKU's SkuID to it, and the [CartItemParentGuid] to the main product it's being added to.
And in there, to the ShoppingCartItemInfo cartItem, set the cartItem.CartItemParentGUID to the parent's cart item guid.