It seems that both .igi and .pigs files are used by Indigo Render and if any software can somewhat convert or transform one to another, it would be this program. Sadly, we haven't found any relevant information about this conversion or whenever it is actually useful or even possible. If you know anything, please let us know in the discussion below.
Convert Indigo Renderer picture image file to Indigo Renderer compressed scene file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert igi to pigs on Windows
A physically based and photorealistic renderer
Convert from igi 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.