Yeah, if you just mix it and shake the fuck out of that, it WILL end up with hot spots, and with a drug of fentanyls potency, it will take but time, for some poor bastard to end up snorting that one wee chunk of 10mg pure fent, etc, then you have a corpse you need to worry about cutting up and disposing of.
it could be dissolved in solution with your cut though, and then spray-dried *GAS MASK, BOILER SUIT AND GLOVES DAMMIT*, since the two are in solution when the droplets are atomised, then it will be evenly mixed.
Just to be thorough I would then ball mill the shit out of it for a week, but that is just me.