I understood his question, but i thought this will be a simple workaround. and no harm done if it didn’t work, he might give it a try.
[ofcourse after installing the wordpress files normal way first]
Won’t index.php code use functions calls to excute all other related wordpress files if it was implemented this way inside his already made html file?
Write the following line inside your .htaccess file on your server(if u don’t have one create one on text file, upload to root directory and rename it to .htaccess):
AddType application/x-httpd-php .html
then copy and paste the code inside the wordpress index.php into ur already existed html file.
Hope this helps