I've mixed strains of cannabis together to tailor a 'buzz', but never other substances that would be destroyed by the combustion. So what 'other substances' are you thinking of?
“Sprayed, dipped, dusted....”
Take your pick. Grind something up into a fine powder mix it with a liquid and spray it on the crop or on the plant that your bagging up and sell it at a higher price.
It was a good way to make Bad Mexican weed good. They could have been using horse tranquilizer then I’m sure if they’re doing it now it would probably be Fentanyl and that would be disastrous.