Can I connect a HDMI from a Wacom Cintiq to my iMac?

You'll need one of these:

http://www.apple.com/shop/product/HFQL2VC/A/moshi-mini-displayport-to-hdmi-adapt er-4k

Since the 5K iMac does not have an HDMI port.The adapter plugs into one of the 2 thunderbolt ports on the back.

You will need to play with the resolution settings once connected as I don't believe the Cintiq can handle the 5K res of the iMac.

No. HDMI ports on Macs any Mac are output only.

But I think you got it backwards. The Cintiq generally will show what is on your desktop on its screen, not the other way around.

You use the Cintiq as basically an additional screen for your computer, so you can draw on it more comfortably. Your Image App on your deksotp will appear on the Cintqs screen. So you are already seeing what is on its screen on your computer.

Unless you are using the Companion, viewing what is on the Cintq on your computer screen does not really make sense.

There's nothing on it. Its mirroring your computer's screen.

Well you could physically do it but if you are wanting the pad display to show on your iMac the answer is no.

The MacBook does not have the ability to act as a display for the pad.