4.3 From other systems
If you have a website built by other applications or systems, your best way to go may be to import your website to WordPress first, export it to Jekyll, and clean up the Markdown files. You can try to search for solutions like “how to import blogger.com to WordPress” or “how to import Tumblr to WordPress”.
If you are very familiar with web scraping techniques, you can also scrape the HTML pages of your website, and convert them to Markdown via Pandoc, e.g.,
rmarkdown::pandoc_convert( 'foo.html', to = 'markdown', output = 'foo.md' )
I have actually tried this way on a website, but was not satisfied, since I still had to heavily clean up the Markdown files. If your website is simpler, this approach may work better for you.