This likely is not correct, but something in this manner should work.
@="\"C:\\Program Files\\Application\\HxD.exe\" \"%1\""
Doing (something like) this should add a context menu (but that may not actually be there in what I've posted?) that should display for every file type, allowing you to open the file with HxD.
Vim does this providing an "Edit with Vim" context menu for all file types.
(There are other Registry keys where the same sort of thing can also be accomplished.)
Someone more knowledgeable should be able come up with the actual working solution.