Yes, just doubleclick on it. It will open up in a HaikuDepot window and then you can click on “Install”. The only catch is if that hpkg has dependencies that need to be installed first, in which case it will put up a dialog.
Alternatively, just move (move, don’t copy) the hpkg to /boot/system/packages
To have a local folder recognised as an actual repository, however, would take some extra steps. First, create a repository on your local disk, then point pkgman to it. . See this recent thread on how to create a repo from a bunch of hpkgs . I’m not entirely sure that pkgman add-repo would recognise a localhost:// address, mostly becaue I’ve never tried. Anyone?.