If the the gauges in P3D work the same as FSX all you need to do is find the HSI you want to use and open that plane's CAB file. (Or folder if it's only that.) Find the xml file and images associated with the HSI and copy/paste them in a folder on your desktop. (Let's name that folder HSI.)then drop that folder in the panel folder of the plane you want to use it in. Open the panel config and add this line to a section in the CFG file. (I usually go to the [Virtual Cockpit 01] group.) It is pretty much the same concept as adding a texture to your plane. You have the folder you created (HSI) and the xml file you copied (maybe it was called HSI.xml). Find the HSI heading in the section you desire and swap it with the new one like this...
gauge10=Flying Wing Gauges!radio_compass, 550,84,98,125
gauge11=Flying Wing Gauges!turn_and_bank, 654,84,98,125
gauge12=Flying Wing Gauges!HorizontalSituIndicator, 447,84,98,125 <---------
gauge13=Flying Wing Gauges!directional indicadtor, 280,84,77,125
gauge10=Flying Wing Gauges!radio_compass, 550,84,98,125
gauge11=Flying Wing Gauges!turn_and_bank, 654,84,98,125
gauge12=HSI!HSI, 447,84,98,125 <---------
gauge13=Flying Wing Gauges!directional indicadtor, 280,84,77,125
*Don't forget the exlamation mark or comma or the gauge won't load!*
You might have to tweak the numbers a bit to place it just how you want it. The first two are the horizontal and vertical location on the panel (I always forget which is which, so you'll need to experiment) and the last two numbers are the vertical and horizontal size of the gauge. On a round instrument they should be close to equal. (Sometimes a panel may be broken up into sections so if you want to verify the gauge you are about to change you can turn it off with the double slashes // and that gauge will no longer be visible in the VC or panel. These may seem like long instructions but if you're used to adding textures this is almost as easy as that.
GOOD LUCK, AND HAVE FUN!!! (It IS fun once you get the hang of it. I had a great time fixing the gauges for the VC panel on the H8K here!!)