Yes, in the wild all dogs would eat the same high-protein diet. Akela already includes the joint support ingredients glucosamine, chondroitin sulphate and MSM. Senior dogs are more prone to weight-gain so you may have to reduce their daily feeding amount slightly, a guide amount is given on our feeding guide.
All of our recipes are suitable from weaning to old age. Other inferior brands get away with making poor quality "adult" foods but can't do so for puppies or seniors to the same extent, hence the "lifestage" phenomenon in the supermarket brands. We recommend our Scottish Salmon dry kibble recipe for seniors as it has the highest levels of joint support. In terms of a "filler", if spreading the daily feed out into several smaller portions doesn't work, then I would recommend a limited amount of celery (cut into small pieces with the leaves removed) or other low calory vegetables rather than using inferior kibble. If you are set on feeding a senior labelled food then I would recommend our sister brand, Country Kibble:
Please bear in mind that although it is superior than many senior foods on the market, it still has a lower level of joint support than our Akela Scottish Salmon product.