Frank's suggestion is a good one, but here's what I do when I want to ritard the ending of a song during a live arranger playing performance with my KN5000:
1) I pre-record just "tempo changes" to the Control (CTL) track in either step or real time for the ending bars of a song. It is very IMPORTANT that you accurately keep track of the song length in bars so you know exactly when the tempo ritard will begin.
2) I then set the sequencer to "play mode" and set the "sycro-start button" to ON. The CTL track (which contains the tempo changes) should be the ONLY track in PLAY mode. All other tracks should be OFF.
3) Begin playing your song in any of the APC modes "one-finger", "fingered", or "pianist" modes and the sequencer CTL trk also begins playing (but with no sound as the only data contained are tempo changes) and when the ending bars of the song are reached, the song will slow down in a "smooth" ritard as you recorded it earlier. All APC features (accompaniment chord recognition etc.) & panel memory "sound changes, etc " will continue to function normally.
Though this method is not 100% live arranger playing, it is much easier than trying to reach for the Tempo Wheel (and attempting a slow ritard) or trying to synchronize pressing the foot pedal to trigger panel memory increments (gradually descreased tempos stored in ascending panel memories) while concurrrently having to focus on playing the keyboard.
This setup is not well suited for spontaneous playing but works great for specific songs you may want to have setup for this special ritard ending situation. You can also add tempo changes (acceleration/ritard) etc in other parts of the song as well.
Happy arranger keyboarding,