Hello,
Unfortunately, there's currently no way to migrate from a Windows-based control panel to cPanel, so this would have to be performed manually for the site migrations, although we would be happy to try to answer the questions you might have.
Probably the best course of action would be to create each account on the cPanel machine, then to FTP download and FTP upload the contents for the sites.
Next, what is the email server being used for email services?
Of note, MSSQL databases will not be able to be transferred as cPanel Linux supports MySQL and PostgreSQL.
Thanks!