You can create an extra waypoint close to the airfield and make the leg from that waypoint to the airfield at a particular altitude. SkyDemon will then implement a descent at the end of the leg before, to try to ensure you end up at that altitude before that waypoint.
SkyDemon only currently allows for one descent performance profile in your aircraft so you may need to either change that, or use an alternative aircraft profile where a rapid descent is required, if you want the performance planning to be exact.