Sure it's possible, but it's not a good idea, unless you want to find out the whole point of Hyperglide.

HG cogs are designed to sit next to the cogs they come with; your shifting will suck. Also, swapping the 15 for a 16 rather than inserting a 16 between the 15 and 17 will just result in an even bigger jump to the 14, which will majorly suck cause you're going faster, facing exponentially more wind resistance, and the last thing you want is a bigger gap (which you know the feeling of, since you're missing a 16).

What you're proposing is 12, 13, 14, 16, 17, 19, 21, 24, 27, right? This is never seen for a reason. That reason is that ideally, gaps between ratios should get smaller as they get higher, due to the exponential nature of wind resistance.

I'd suggest you buy a 12-23 and a 13t top gear and make up a 13-27, or skip the 13t and do 12-24.

Sheldon reckons just jamming a cog that doesn't belong in there is no big deal, but he seems pretty slap-dash on this subject, inviting folks to mix and match cogs of different thicknesses.

http://www.sheldonbrown.com/k7.html#custom

http://www.sheldonbrown.com/harris/k7.html#9cassettes

IME it works like crap if you don't preserve the proper relationships between HG cogs.