Hi,
This is indeed an unusual request. Do you have Type 1 DM, and how well is your diabetes controlled? If your diabetes is not well controlled this could be the cause of your not being able to gain weight. If you have Type 1 diabetes, you may also be at risk for other diseases that can contribute to poor weight gain, like Celiac disease.
The other question is, why do you want to gain weight?
You can some nutrient dense foods like nuts which have good fats and proteins without too high a carb load as part of your diet. Weight bearing exercise is great as a general rule. Meeting with a dietician may be very helpful in this situation.
Take care.
Hope all goes well...