Yes, any applicant to a US college or university can sign up for a Coalition account and apply to one or more colleges.
Once you have an account and you begin filling out your profile, you are asked to specify your high school. Many international high schools are already in the Coalition system, and it will be listed as an option in your search results. If your high school is not listed, simply select "School not found" and enter your high school information manually.
When you begin a college application, you are asked to specify what applicant type you are, and at that time, you should select the appropriate application type (such as International Freshman or International Transfer).