If it’s a static html-site, with an index.html or index.htm, it’s quite easy actually.
This is the way I work in this case: first, of course, I check if there are no files or folders that conflict with the modx structure and I make a backup of the site.
I then just upload the modx install files and add this line to the .htaccess file:
DirectoryIndex index.htm index.html index.php
This way index.php will only be called if there’s no index.htm and no index.html.
I allways use friendly aliases, so I name the alias of the homepage in modx something like ’test’. To test your modx site, you can call it with www.example.com/test.html.
When your’e finished, you can bring your new site online within seconds. Just rename your alias ’test’ to ’index’ and rename index.html to index.old, and your new site is running
After a few days or weeks, you can clean up and delete all the old files.