Fat is the most calorie-dense macronutrient. Fat has 9 calories per gram; protein and carbohydrate each have 4 calories per gram. I would say, just add enough oil to the Soylent to give the desired amount of calories, as long as it’s still palatable. For instance, 2/3 cup of canola oil (146 grams) has 1291 calories. Add that to the 2010 calories of Soylent (including the fish oil) and you’ll have 3301 calories. That would be a lot of oil, it might not be pleasant to drink. You might have to experiment with different ingredients to get something that tastes good.
Something to be concerned about is, even though her metabolism is increased and she needs more calories, how is her absorption of nutrients? The pouch of Soylent has over 100g of protein; that’s already an awful lot of protein for a 5 year old, and too much protein can be hard on the kidneys. If you add Cyto Gainer and dairy for even more protein, and her kidneys have to process all that, it might be too much.
Soylent contains 100% of the nutrients recommended for a healthy adult. The micronutrients (vitamins and minerals) in Soylent, along with added micronutrients in Cyto Gainer, might put her above the UI (upper intake) for some nutrients. As a child, her maximum safe level for some nutrients is lower than an adult’s.
So those might be some considerations for using plain oil to reach the calorie target, instead of additional nutrition supplements or dairy. You also might consider sugar, which is pure carbohydrate (and might make the whole thing taste more appealing to her). 1-2/3 cups of table sugar (334 grams) contains 1293 calories. Some combination of oil and sugar might be the most palatable.
I think it might be best to consult a registered dietitian and/or a CP specialist about this. There are medical nutrition products (usually, a shake in a can) tailored to meet a variety of special circumstances. Hopefully, her insurance would cover a medical food product.
Also, prepared Soylent is 2 liters, so if you add 3 cups of milk and a cup of ice cream, you’ll wind up with more like 3 liters, so if you do go that route you’ll want to calculate for the 3300 calories being in 3 liters, not 2 liters. Adding oil and/or sugar would mean 3300 calories in closer to 2 liters, much more dense and much less for her to drink.