If time is of the essence, there is an easy workaround at least.
Assuming that the epix file looks right in Piranesi in terms of the materials and depth channels, all you have to do is save your rendered image from ArchiCAD and import it into Piranesi as the new RGB image. "Re-Render" in Piranesi to save the new RGB as the Restore RGB, and you will then have the epix file that you wanted in the first place.
Another plus of this technique, depending on what you're going to do in Piranesi, is that you can do some work in Photoshop on the rendering first before making it the new epix RGB.
Cheers,
Karl
AC 28 USA and earlier • macOS Sequoia 15.3.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB