Although this conversion is probably doable, it does not make much sense to do so. You would basically extract a picture from .igz file only to repack it again in TGZ format, which would make it not usable in graphics programs that usually work with .igz graphics.
Convert Gzipped image file to Gzip compressed TAR archive.
We found one software available in the database for this conversion.
Linux/Unix software - convert igz to tgz on Linux
File compression utility for Unix
Convert from igz file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.