Can someone tell me how to chroot FTP users with Virtualmin, if it is possible.
Have you taken a look at this thread?
What you want is not actually a chroot. You want the DefaultRoot setting in ProFTPd.
You can set that in Virtualmin by browsing to Webmin>Servers>ProFTPd Server>Files and Directories, and set the option labeled "Limit users to directories" to "Home directory". Save it, and click "Apply changes" in the upper right corner.