When using grapes, making port-style is essentially letting about half
of the sugar ferment, then topping up with spirit (for example Taylor's
uses a 77 or 78% grape spirit, but many home-winemakers use neutral
grain spirits) to bring the alcohol up to 20%.

I have no idea if it would work well using a kit, as they are generally
adjusted to be finished dry, but I imagine that if nothing else it
would be delicious and interesting ! What more can you ask for !