Version: (using KDE 4.0.82) Installed from: KDE4Daily provide option to make popup notification windows "sticky" (with close widget) Because not every user is fast reader. So with such option turned on, every popup notification should hang till end of the world. Until user clicks on it. While close button -- because then there is less teaching. Users already know that in order to close something they should click on close button.
Not exactly what was requested, but in Plasma 5.14 (and maybe earlier as well), there is a Notification history, so you can look at the missed popup notifications.
I'm afraid this is not doable, sorry.