The scripts API is somewhat limited, and probably won’t give you the kind of control you’d need to add stuff to the UI during install.
The actual Install Scripts code is in scripts-lib.pl and script_form.cgi and script_install.cgi (all in the virtual-server directory within the Webmin install directory).
If you’d like to contribute back your changes, you can send us patches via the ticket tracker (no guarantee they’ll be included, as we’re currently doing some renovations to the Install Scripts as well).