Yes, you can actually duplicate existing template easily and make any changes there directly. Let's say you wish to get duplicate template for 'template'. All files related to template are stored in /usr/local/sitebuilder/htdocs/templates/template directory. You need to copy files and change template name in info.xml file, so
# cp -rp /usr/local/sitebuilder/htdocs/templates/template /usr/local/sitebuilder/htdocs/templates/template_duplicate
After that edit file /usr/local/sitebuilder/htdocs/templates/template_duplicate/info.xml and update template id from:
<template id="template" to <template id="template_duplicate"
Id should be the same as directory name. After that run following command to update list of templates in Sitebuilder database:
# sb_config --update_templates
Do not forget to add new template into appropriate plan so it could be available through Wizard.
As soon as template is ready you can go into /usr/local/sitebuilder/htdocs/templates/template_duplicate directory and make any changes you need there.
Hope it helps.