Ok, I thought you wanted Rsr2Png to generate PNG when RSR exists, and RSR when PNg exists, at the same time. apparently what you are asking for is just recursive conversion in the folder substrcture, am I right ? So, this feature already exists in P3dO pro. With P3dO pro Rsr2Png can convert thumbs of a whole library at once. You have to select the desired runtime, click "Explore subfolders", then run Rsr2Png.

Yarp - author of P3DO Organizer for Poser