If you're talking about an HP notebook or computer, that HDMI port would have been an output port, not input.
If you're talking about an HP monitor and you're wanting to connect external devices to your iMac to view them, then NO, there is no input for viewing external devices on your iMac screen. You will need a video capture card to do that as with any of the "all-in-one" systems whether by Apple, Dell & others that I have seen.
The most popular of these for the Mac are the TV tuner cards by El Gato. But there are several brands out there now including one by the company one of our mods works for. (Aptmunich, now's your chance - I never can remember the name of yours, plus I'm at work and don't have my bookmarks.) You can find them with a quick google.
