Hi, welcome to the forum
If you already got html version of notes, that's great. You need to convert them to .md and Joplin should import it with no problem after that.
To convert from html to markdown (.md), you can use pandoc. Install pandoc, navigate (with a terminal) to the folder with your html notes and run this:
for f in *.html ; do pandoc ${f} -f html -t markdown -s -o ${f}.md ; done