It’s possible, but it requires a bit of tinkering. The easiest way to do it is if your main site’s code is clean and readable.
After that, make a copy of the standard theme and start editing. Replace values in the header.php with those from your own site. Then do the same for index.php and footer.php
My site is still a major work in progress, but you can see how I’m progressing from html to wordpress here:
html template: https://themadmanproject.net//School/ITM445/project_wip/index2.htm
WordPress WIP:
https://www.themadmanproject.net
There’s still a lot of programming to be done on the site template and design, but hopefully it’ll help
