I had to do this for another device a while back and remember it being a little awkward, I think you can do it using the "Edit End Sequence" for the activity in the Harmony app and use that to add a second "Off" command. However, I seem to remember that annoyingly it doesn't let you just add a second off command into the sequence and what I add to do was learn the "Off" infra-red command but under a different name and then add that new command into the sequence, with a delay after the first "Off" command.
