Yes, you can use impermanence as a separate reflection. In the Seven Reflections I did separate death from impermanence. You can do a whole section of reflecting on death, you can do a whole hour of reflecting on impermanence. That is totally ok, you can do each as a separate reflection.
Now, as to impermanence, resisting impermanence being the cause for Dukkha, yes, by all means, reflect in that way. Reflect on how many times in your life things were impermanent and you created Dukkha in your reaction. Here is a typical example, I am 15 years old and one of the tallest kids in the class. Playing basketball I was in the front, a position called the Forward, because the tallest play in the front. But when I was 15 years old, I stopped growing. The very next year I was not one of the tallest kids in the class, in fact, quite a few where getting taller than me. All of a sudden I was told by my basketball coach I have to play at the back as a Guard. Impermanence. I suffered. I couldn't handle that position, I quit basketball. So resistance to impermanence often creates Dukkha. Now, look at some of those men here, all you have to do is have a look in the mirror, you women, too, look there is impermanence. Look at the grey, impermanence. The wrinkles - that goes for everybody, right? So all of these things are impermanent. Look at how many times in your life you start to resist it, you don't want it, and how it then creates Dukkha for yourself. It is very good to reflect on that.