Well, it's already seeded in the note that is discovered in the Water Cave. Also, the majority of the hooks presented at the start of H2, assume that the players are in Winterhaven. So, you have plenty of "out of the box" hooks. The "Keep on the Shadowfell Side Treks" article from last month's Dungeon magazine also includes a bonus encounter with the slavers.
I played up the dangers of the roads to Winterhaven in the adventure pre-amble, and initial adventure plot hooks. This not only set up the kobold ambushes, but will also seed H2.