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Yola sitebuilder is a dedicated system which doesn't allow for other building systems. The sites must be built with the Yola sitebuilder to work.
Having said that you can use CSS and generated content HTML to add to your site. You can't use other third party templates for building a site with Yola.
So you can't upload a third party website building program for use with Yola. It is disappointing I know but the system is designed for easy use which can be fully supported by the CS people of Yola whereas third party software is just too big a frontier plus the potential security issues and platform design to enable this.