You haven’t given much detail about your problem at all. However, this seems a solid constraint:
in which the inbound and outbound routes are predefined
So, being me, I’d set a distance/time matrix up in such a way that it’s only possible to go on one route (i.e. the vast majority of distances/times between location pairs are effectively infinite, forcing a particular route). That really constraints the problem though, so I don’t know what jsprit is supposed to be optimising on, exactly, after that point. I’m not sure this is a jsprit-type problem.