Can I practice with one of my own urgent life questions as a koan? For instance What is reality? What is life? or What am I? Should I focus on one particular question or can the words change naturally?

Using your life questions as a mantra, your personal koan, the words can change naturally, but do keep your focus on the main question behind all of these questions: WHO is it who perceives these questions? WHO is it who is asking ? From there your mind can truly open.