You can import any files. #import is just a preprocessor directive that basically says: "Include the contents of the file in which I will tell you in the current file." There are some defenders, but import does just that - it imports the file that you tell it to.
Importing a header with a .h file is probably more a convenience / convention (please don't beat me up for this, I just guess) unless there is some C / C ++ standard that says otherwise.