Best way to import a complex site to multisite subsite

Hi,

I have the website trainervanacker.com and would like to import the entire website as a subsite under fitguana.com

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 trainervanacker.com under fitguana.com (multisite) with domain mapping.

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

Thank you!

Sander