I encountered the same problem and managed to write a custom posterizeTime() expression.<div>
Just use .vaueAtTime(newT); on your desired property.
newFramerate = 5; and frameOffset = 2; will change the values on frame 2, 7, 12, 17, 22, …
I hope that works for you, too!
Jakob
// Custom posterizeTime()
//
// Use .valueAtTime(newT) on your desired property
// and set your new frame Rate and your Offset value
var newFramerate = 5; //set your new Framerate in frames per second here
var frameOffset = 2; //set your offset in frame here
var frameRate = 1/thisComp.frameDuration;
var t = (time*frameRate);
var newT = Math.floor((t-frameOffset)/(frameRate/newFramerate));
newT = newT*(frameRate/newFramerate) + frameOffset;
newT /= frameRate;
transform.position.valueAtTime(newT);
</div>