As someone mentioned there are already sewers in Hempstead. You could do something as simple as have a few entrances/exits added. One that maybe opens in an alley near a crafthall or the merchant district, maybe one that has a crawlspace to the fields outside town. Adding transitions with nifty descriptions does cost much in the module.
In Prantz, a shadier temple there is already underground. So why not have an extra exit or two from there as well. Maybe one that lands outside the city and another than gets you in another part of town.