Increasing height of 27" iMac
We can separate the possible solutions into intrinsic and extrinsic solutions, as well as sitting/standing solutions.

Intrinsic solutions (ones that will not look like an afterthought):


You can "[b]uy [an] iMac with Built-in VESA Mount Adapter". You will not see this option on the usual checkout lane, but Google locates it.


The 27" iMac weighs 21 pounds (9.54 kg). This could make it challenging to couple it with some random monitor arm on the market.

Extrinsic solutions (an additional base with a potentially jarring look):


Rain Design mBase Stand for 27" iMac, $69.95
Bretford MobilePro Desk Mount, $179.95
A custom solution that matches the desk rather than the iMac.
Satechi produces a plastic and an aluminum stand. They're not adjustable, but breakage is less of an issue than with glass.
Fitueyes clear Computer Monitor Riser 4.7" (thanks, Allan)
Halter Acrylic Monitor Stand / Monitor Riser 3.675"H


The trouble here may be that you're designing a desk for just one person. If you are pursuing a more fluid ambiance at an office and wish to accommodate someone ranging from, say, 5ft to 6ft 2", the presence of a base may not be too convenient.

Sit/stand solutions


Ikea sells a manual and an electric adjustable desk. They are sitting/standing desks, and so they are a much more general solution for problems in ergonomics. To use while sitting, you will need to look for a really deep keyboard tray (or possibly get a second lower table to position underneath, if you won't find that setup particularly jarring).
Ergotron WorkFit-A, Sit-Stand Workstation, $595
Varidesk standing desk Pro Plus, $395 to $595.


Note that neither the Ergotron nor the Varidesk solve the problem since the height difference between the keyboard and the monitor platform is constant. A monitor pedestal will still be needed.

To conclude, your best bet may be to look for a sit/stand desk (google/bing) and hook an adjustable keyboard tray (google/bing) underneath. Unfortunately that's not a cheap option.