I came here to ask this very same question. I have fully transitioned from using Reason as my main DAW and found it to be remiss of Presonus to have not included this feature. Both the NN-XT and NN-19 allows you to enable this feature and these samplers are older than my nephew who is long graduated from college. The transient detection in Studio One is good, but dare I say, far from pinpoint accurate. So when you import sliced audio into Sample One XT, the chances for having to adjust start and end points are as high as Uncle Sam taking a cut out of my weekly paycheck (unless I move to Texas).
The good guys at Presonus do an excellent job. So although this feature is likely at the very bottom of the pile, I'm crossing my fingers this is included within the next update or 2....should be easy as pie in a Little Debbie snacks factory.