I have a 2001 DTS with the same Nav system, and am equally frustrated with GM abandoning Denso (and Denso retaliating by refusing to produce updates).
I ripped one of the CDs and found that it is based on the KWI mapping scheme. KWI is/was a consortium of Japanese Nav producers who wanted to reduce the cost of updating maps by using a common modular system.
In essence, there are a few software files that provide the drivers to interface with your hardware and a few data files that provide the map underlay, the intersection/turn information, the street address index and the points of interest index.
This means that, assuming that Infiniti is using the KWI scheme, we could take the vehicle specific information from the Cadillac disk and the map files from the Infiniti disk and burn these files onto a blank CD-R.
I am currently traveling, so I do not have access to my Caddy's disk 5 to advise you which files are which. If you post the directory lists from both disk 5s, we can decide together which files to pull from each disk.