Searing a roast before putting it in the crockpot is not strictly necessary, especially if you are making a slow-cooked Cuban pork shoulder roast similar to carnitas. The purpose of searing meat before slow-cooking is to develop a crust and enhance the flavor by browning the surface. While searing can add some depth to the final dish, it is not essential, and you can still achieve delicious results without searing.
In the case of Cuban pork shoulder roast (or carnitas), the slow-cooking process itself will break down the tough fibers of the pork and render it tender and flavorful. The long, slow cooking time allows the meat to become tender and develop its characteristic taste, so it will be delicious even without searing.
However, if you prefer the added depth of flavor that comes with searing, you can certainly do so before placing the pork shoulder in the crockpot. Simply heat a skillet or pan over high heat, add a little oil, and sear the pork shoulder on all sides until it gets a nice brown crust. Then, transfer the seared pork shoulder to the crockpot and proceed with the rest of the recipe.
Ultimately, whether or not to sear the meat before slow-cooking is a matter of personal preference. If you're short on time or prefer a more straightforward approach, skipping the searing step won't greatly impact the final result. The most critical factors for a successful Cuban pork shoulder roast (or carnitas) are the spices, seasonings, and the slow-cooking process itself.