Hello, @IP,
Actually, Yes. Importing the demo will override all the existing content. But as you said that you want to import only some pages, and sections of other demos of our theme without resetting the database and other site content so for this you can use the XSTORE Studio feature of our theme. Here is an article to learn about it and understand how it works: https://xstore.helpscoutdocs.com/article/148-xstudio-feature-of-xstore-wordpress-theme
Further, While importing a demo it also, allows you to select the specific content of the demo you want to import see the image: https://i.is.cc/kAzLydM.png you can also, check this feature as well. The choice is yours now.
Regards 8Themes Team.