Can clonazepam be used to treat insomnia due to ADHD stimulant medications?

Not the best idea. Clonazapam can be habit forming and addictive. It's in a class of medications called sedative-hypnotics or downers. Stimulant medications can be considered uppers. So you take an upper in the morning and a downer at night? It doesn't seem wise, right. One can build up a tolerance to clonazapam, and over time, require higher and higher doses, leading to other problems like memory loss. Avoid it.
Need attention. When you discuss this matter with your doctor, just keep in mind that Clonazepam is habit forming and if at one point you want to stop it, there will be some problem with a long process of tapering.
Depends. Many with adhd have sleep problems, either with the disorder or as a result of meds, especially stimulants. Sleep problems tend to be chronic, so using meds to treat the sleep issues requires careful thought about longterm pluses and minues. I try to use nonmedication approaches first; if meds prove necessary will choose something that helps another problem like adhd or anxiety if possible.
Yes, among others. Yes, Clonazepam can be used to treat insomnia with adhd medications. Often, when a person with adhd reports insomnia, turns out that they haven't taken a stimulant for hours before bed, and have always had insomnia. Sometimes the problem has been that they need a dose closer to bedtime so they can shift attention to going to sleep. People with adhd often have an opposite effect from stimulants.