Not sure what board you are using, but if memory serves me correct there is a style in the Tyros style for New York New York that has the ritardando built into the ending.

Alternatively just select the ending for the style you are using, and from the edit screen choose overdub, (Or whatever your keyboard calls it) then when the ending starts, slowly vary the tempo as required. (Don’t forget to save the modified style afterwards)

Hope this helps

Bill