I am planning on tying it in, after the first AP, and before the second one.
The PCs, to complete their notes on finding Tazion, need to talk to an old explorer/riverman, who is supposed to be in Port Freedom (think that is the name, the port south of Eleder, where the rivers actually reach the sea).
Once at Port Freedom, the dragonnel attack occurs. The merchant knows where to find the explorer, but they are far upstream. He is willing to take them there, as he is going there anyways, but they will have to pay for their passage by being guards.
His ship is powered by an ancient Azlanti artifact, one of several that were found a few years ago. This is to fit in with the campaign, and explain why the fuel creature is only found in the Expanse area.
Once at the station, the riverboat would be trapped due to the siege like atmosphere with the native elves (or some other locals - not sure yet). The old explorer is nowhere to be found (killed and animated by Petrune). The PCs are offered employment as guards at the station, for the duration of the siege.
After the final encounter, the PCs will find the old explorer's log book in Petruna's effects, allowing them to complete their notes.
Likely the PCs will irritate the Rivermen's Guild, which will not allow them to use the riverboats to get part way to Tazion, which is why they are going overland. And if the PCs end up on friendly terms, then the explantion will be that there are too many people in the expedition to fit on the available boats.