Can a penalty still be added after the stroke-play competition has closed?

Normally no. A penalty must not be added or corrected once the competition has closed, which is when the result is final in the way the Committee set, or, in stroke-play qualifying before match play, when the player has teed off in their first match (Rule 20.2e(2)).

But a player must be disqualified even after the close if they:

• Returned a score for any hole lower than actually taken, unless it was only because of penalty strokes they didn't know about before the close
• Knew before the close that they had breached any other Rule with a disqualification penalty
• Agreed with another player to ignore a Rule or penalty they knew applied

The Committee may also disqualify for serious misconduct after the close (Rule 1.2).

In match play, there's no time limit on disqualifying for serious misconduct, or for deliberately failing to apply a penalty or agreeing to ignore a Rule (Rule 20.2e(1)).