Yes, it would be hard to create a new User Defined Language…
For using existing HTML highlight, you can add the liquid extension to langs.xml file in the Language name="html" part and you can add some more keywords inside <Keywords name="instre1"> so these are highlighted.
Some reading for creating a new User Defined Language:
http://docs.notepad-plus-plus.org/index.php/User_Defined_Languages
http://docs.notepad-plus-plus.org/index.php/User_Defined_Language_Files
http://superuser.com/questions/160925/adding-a-new-language-to-notepad
http://stackoverflow.com/questions/12574121/adding-a-user-defined-language-in-notepad
http://weblogs.asp.net/jongalloway/creating-a-user-defined-language-in-notepad