Never mix anything with your powder, never mix powders.
Filler is put on top of the powder to bring the ball closer to the top of the cylinder. It can be done with wads but sometime its half an inch or more that needs to be made up so an inert granular filer is used.
Theres quite a few things that can be used, semolina, cous cous, coarse ground rice, flaked rice etc but dont use sugar as it is an explosive in certain circumstances. What we are looking for is something clean in its use but nothing plastic because molten plastic can gum up the barrel.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane