What do you mean by 100% powdered - until what age?
But I doubt it because they're driven by instinct to hunt. If they don't eat during their first few days, they will die. The ones that do take them will only be able to eat powdered for the first week or so.
Betta fish are "hunting" fish, they like to chase their food and play with it, its good stimulation for them!giving them powdered fry food if I'm honest you might as well give them a death sentence!(no offence intended) even if one or two of them take it the rest will die and then the ones that do take it will never grow well and will be weak and not have a long life.I would advise you NOT to try it!