Best way to import a complex site to multisite subsite


I have the website and would like to import the entire website as a subsite under

Once it's imported, I want to change the theme to upfront and make a lot of changes but have the basic website intact to save a lot of time!

Later, I'll change the domain name back to under (multisite) with domain mapping.

My question is, how do I import the website structure and files best without having to start over?

Thank you!