Hi! With about ALL animals, u must introduce more food SLOWLY. Equines may get deadly sick from to rapid change in diet, however I've never heard of this in suggies-though I
have heard that you must give new foods to them slowly, in baby steps. Try mixing it into their food bowls, just the tiniest thing in the start, and a bit more when they eat that, a bit more, and so on. Slowly but surely mixing in new foods is the key to non-rejection in many animlas, and this might very well work, though as you say the may just not want it.
BUT! Since it't called a treat, it might be health risky to provide them with to much! I am not experienced with gliderade and do not know how much is ok myself, so I trust you ownership-qualities that you will make sure you know this before you attempt to feed them more then just sometimes!