Welcome to the forums.
Yes, you can import HTML files, and yes, RoboHelp has
templates. Importing an HTML file to use as a template requires an
extra step because RH's templates have a file extension of .htt.
With your HTML file open in an editor like Notepad, you can do a
"Save As" and save a copy with a .htt file extension, and then
import it into RH's Templates folder in the Project Manager.
Good luck,
Ben