I have not made this either but just reading the recipe I can't see any reason not to put It it all I to the jug. The reason the potatoes and leeks are in the basket is to allow the water with stock just to just steam the fish. If making the soup there is no need not to just chuck it together.
First I would sauté the onions, garlic AND the leeks before adding the stock and potatoes. Just cook until the potatoes are well done other wise it will be starchy.
Be aware you will not have the slight salmon flavour to your soup but you could use a wee bit of fish stock if that is what you are looking for flavour wise.
Or you could just make leek and potato soup from the EDC page 83!
Hope it helps.
Gert