metronic only deals with rendering UI for dashboard/admin panels/any type of page really, so if you want to use it on top of Twill, you're going to have to integrate everything to the internals of Twill, which means a crazy amount of work depending on the features you'll need to recreate using those components. For example, how do you envision building Twill's block editor using Metronic? What does the client likes more about it? If they are just looking at the dashboard modules, that's not really the core of a CMS, tables and forms are more fundamental aspects.