You can purchase a commercial floor filler (such as Goop-on) or purchase wood dough base from a commercial wood floor dealer, or perhaps a paint supplier. No not use sanding dust out of a belt or drum sander unless it is only from the last pass, as it is to stringy. You are better off to use the sanding dust out of the Edger sander. Do not try to fill gaps of more than 1/16" on an old floor or you will be asking for problems.