308 votesMoose commented
An easy way would be to have a product attribute that marks an item on a kit, and for each kit item, provide a way to add the constituent SKUs and quantities as attributes of the "parent" kit item (like adding several aliases).
The only other thing necessary would be to add an automation that modifies the order items of any order containing a kit product by removing (or zeroing) the kit item, and adding each of the constituents as individual order items as the order is imported.
There is already similar functionality to fire a webhook based on specific order items, so I imagine it something like the webhook fires on kit items, the endpoint accepts the SKU of the kit and does a database lookup of what's in the kit, then modifies the order items via the API, returning the contents of the kit to shipstation as JSON or XML for shipstation to interpret as order items.
It doesn't look like there's a way to modify the order items though the API currently, or I'd have this implemented already. It doesn't seem like it would be too difficult for the shipstation developers to add, though.