I’m not sure if this is what you ment. But inside the A32NX folder, there is a “CUSTOMIZE” folder, with a .psd file. In there you find the cockpit reg plate which you can change. You have to convert it to a .dds file, when done (with Photoshop or Paint.net). Rename it A320NEO_COCKPIT_DECALSALPHA_ALBD.PNG.DDS (as Simson166 described) and copy it to the A32NX texture folder.
In general: I think I saw an entry in the Panel.cfg to implement the MSFS reg in the cockpit, but I never tried it out. This may work in default planes, but the A32NX reg plate is at a different position than the default. So it wouldn’t work anyway.