Mix chili powder, cumin, and paprika in a small bowl. Rub into the chuck roast on both sides (you might have some leftover). Add olive oil to a large nonstick pan and heat over medium-high. Sear meat for 2-3 minutes on each side, until golden brown. Transfer meat to a slow cooker.
Carefully pour the beef stock (or 1 cup water with 1 cube beef bouillon) into the hot pan. Simmer over low heat and scrape up the browned bits. Add the tomato paste and chipotle pepper. Toss in any remaining spices. Boil for 2 minutes, until slightly thickened and pour over the meat. Add onion and garlic. Cook for 6-8 hours on low or 4 hours on high.
When the meat is done, it will shred easily. Transfer the meat to a bowl and discard of the juices in the slow cooker. Shred. Fold in the enchilada sauce and brown sugar. Return meat to the slow cooker and keep on warm until ready to serve.