Hi Coquito-
When I saw the nutrionist at my hospital, she said the fastest way to add calories is to add sugars and fats. She suggested adding a tablespoon of olive or canola oil to soups(it's unsaturated fat that way), you can hardly taste it and it adds 100 calories, or adding a tablespoon of jelly to hot cereal. Peanut butter milkshakes are good too(peanut butter, ice cream, milk- you may need to eat it at room temperature)
Also, avocados are high calorie and slippery.
For the first few weeks after treatment I lived on soup made with top ramen, overcooked asaparagus, eggs and olive oil. Sounds gross but it actually was pretty good.
Mel