You can already do this with the app. When adding a new round, create a new course called, say, prac putt (shorter for practice putt). Then enter data for each “hole,” changing the first shot from “tee” to “green.” As long as your practice session concludes with a hole out, this will compute strokes gained for your practice putting.
You can also do the same with “prac chip”, where if you don’t hole out, you can look at the strokes gained short game results and ignore the strokes gained putting results.