Cochinita pibil is traditionally roasted wrapped in a banana leaf (foil will work as a modern substitute), so there should be enough liquid that the roasting isn't very different from stewing. After it's done roasting, you should let it rest in the liquid as well. If it held together really well, you may want to break it up/shred it a bit first. If you haven't been doing this already, it'll probably be enough to get you what you want.
If there's not enough liquid left for your tastes, you may want to leave more of the marinade in when you roast. If you still can't get enough, you can reserve some marinade separate from the meat at the beginning, and heat it and mix it in at the end. You can also supplement with broth. It's doubtful that you actually need to further cook in the liquid; the roasting should be enough to fully cook the meat, to the point of falling apart.