Welp, I've gnawing on that for a while but I don't have organic ideas to help you get into the story. Right now both groups are about to meet.
Best I came up with was for you to just show up in Harlan and mingle with a few NPCs till the two groups come together.
Or...OR...Nate accidentally hits him with his car.
I know that sounds crazy but hear me out. They're already on the run and Nate probably wouldn't want to stop to help but maybe JJ is less of a dick and tries to heal him. Maybe we could have him follow them after that to figure out what the hell happened or, more complicated, he somehow ends up in the car with them...Maybe Nate doesn't want to stop long enough to let JJ heal him so they take him with them and do it on the way...That one makes less sense but I'm just spitballin'.