Importing formula from other countries also is in that grey area of Customs looking the other way when the quantity is in small amounts for personal use.
There was an old World Health Organization recipe from 2005 for making homemade formula in emergency conditions, but it got pulled in subsequent years due to concerns over giving animal milk to newborns and the risks of contamination and improper measurement in homemade batches.
As I said before, there is more flexibility with older infants over the age of 12 months. That age group you can supplement with solid foods, animal milk, and some brands of toddler formula ( ask a pediatrician). In especially dire circumstances you can also do this for infants that are older than 6 months, but ideally only for short periods and in concert with a pediatrician's guidance for supplementation.
Watering down formula is dangerous as it can lead to severe electrolyte imbalances.
For younger than 6 months, you really should exhaust every means available in regards to breast milk and infant formula. The risks of substitutions are large and the margin for error small.