You've probably answered your own question.
I've always used the lazy (but easily adjusted) method of just extending the length of the item by the required amount (of silence) at the end and if necessary using an envelope. But maybe someone will offer you a better solution.
On edit: What's nice about this method is that you can see what's going on. Simple. Not a setting somewhere, so that, when you come back to it weeks later you don't wonder what the heck's going on!
But if you wish you can try tinkering with the Run FX for nnnnn ms after stopping option in Prefs (Audio Playback) but I prefer the technique outlined above.