Two things I would investigate are meal replacements and powders that bodybuilder types use to bulk up.
Meal replacements such as Ensure can be drunk between meals so you are not only getting the calories and goodness from the food you eat at mealtime; but, you are adding additional calories in the form of the meal replacement.
Most health food stores have tins of powders that are supposed to help bulk up the user. Again, it would be a healthful way to add additional calories to your diet. These powders are mixed in water, milk or juice so it can be real pick-me-up in the afternoon.
If you can't find them in the stores that cater to the European trade in China, you could probably find them online.