Is it possible to extract the he radio image of stock ROM and put it into a flashable zip or IMG to flash over the custom ROM in order to enable volte ?
You act as if we are running a custom radio image on custom ROMs... we aren't.
In general, the radio image is, in 99% of all devices out there, always a unmodified one taken from the stock ROM.
In fact, most custom ROMs don't even touch / flash / modify the radio image in any way, LineageOS for example doesn't even bundle the radio image in the ZIP file, as it assumes your phone already has a working radio image installed (coming from a stock ROM).
So no, the radio image is not the issue at hand.