I used to have a chronically underweight friend years ago (long before Ensure and others), and her doctors insisted she drink one of those instant breakfast shakes in addition to every meal (we were in the military, and she was so on the underweight side that she was afraid of being medical-ed out for it). Now there are many more varieties of this sort of stuff, and finding one that works for you (now, I mean WITH your meals, not in place of) might help.
If you are taking this on, you really should consult with a sports medicine/dietician to make sure you maintain balance while adding the calories.
That said, beans; well, legumes of all sorts are high protein and are good carbs, and go well in any crock pot dish. Plus they are a bargain. Sweet potatoes/yams can be a great addition as well.
Cook everything in the crock pot in a stock/broth, so you have super-rich flavored sauce, and buy some really heavy, whole meal/grain bread to "sop" up the juices of your meal.