Maybe a stupid question, but have you tried sideloading the app? The reason I ask is because while it may not officially support KK, you may still be able to sideload it. T-Mobile VV for example didn't officially support KK until a few weeks ago (and consequently couldn't be installed through GP), but many people, myself included were still able to sideload it and it worked just fine. If you can't sideload it you'd have better luck pressing the app creator to update his app. No developer is ever going to port an outdated version of Android.
Samsung Galaxy S4
Motorola Slvr L7