This is going to be largely dependent on your supplement. In my supplement, there's only language allowing (qualified) feeder drivers to bump down into package during a lay-off. If the feeder driver isn't qualified, the company is required to train him on a route after a certain period during the lay-off.
However, if you're a "cover" feeder driver and don't have a route, any more senior feeder driver can bump you. While every building is different, there aren't too many guys going from feeders back into package cars; I can't imagine there being more than a handful of feeder-qualified package car drivers unless there was a mass-cutting of routes at one point. Everything is going to be more or less dependent on your FT seniority.