This can be done. What you want in your .htaccess is something like
if directory do not exist in root
change root to the wordpress directory
unfortunately I am not familiar with a way to do such a thing with .htaccess
The other option is to map each possible url directory to the wordress directory, but since url directories in wordpress are created dynamically when you add category, page, new post date and more, it will become hard to maintain the .htaccess. Maybe you will have better luck with special treatment to the current site URLs.
And this all assumes that you will not have conflicts in URLs between the old site and the wordress site.