The detergent receptacles on a front-load are there purely for the "convenience" of the owner. There's no "rule" that requires the owner to use liquid soaps and dumping the Borax (or, any other powdered detergent) in with the clothes won't harm them, particularly if they are natural fiber. I agree with the suggestion of diluting the Borax, which is a great booster, and adding it into the wash at the time of loading.