Since you're diabetic, you don't want to start eating a lot in a single meal. You might be better off to add more snacks throughout the day that consist of more fruits and veggies combined with protein and good fats. Have you ever done any research into the glycemic index? You want to eat more of the foods that are lower on the glycemic index. Focus on eating the same things that you would if you were losing weight, but eat them more often throughout the day to increase your daily caloric intake. So, more fruits and veggies, lean protein (chicken, turkey, fish, egg whites), and whole grains... carbs themselves are not bad as long as you stick with mostly whole grains... stay away from the refined stuff.
Also, to put on lean weight, you need to include some form of strength training into your weekly routine, ideally 2-3 days per week, at least 20-30 mins per session. You can do this at home using body weight if going to the gym isn't what you want to do.
