Add enough white flour to to thicken the stuff. Some say "like pancake batter", some say "like peanut butter". I like it in between, like organic peanut butter. The thickening keeps it from running out of the joint. The nightmare that terrorizes epoxy users is the "starved joint".
You can add fumed silica to make epoxy thixotropic (not flowing)or hollow glass or plastic microspheres to make it light and bulk it up. Wood flour is conventional for gluing applications. That's fine sanding dust. You can get it segregated by fiber length if you crave certainty and perfection. White flour is just as good. Whole wheat flour isn't. The extra vitamins make it too greasy and lumpy.