SFS2PET
The SFS2PET Converter lets you convert SFS files to PET files. It runs from PuppyMenu > Utility. It has a GUI interface with file selector and can also be used from the RightClick menu on an SFS file (select Convert SFS-to-PET). Written by RSH.
Screenshot
Installation
Install an appropriate software package from the forum thread
Also on the Wiki
PETS2SFSGUI - combine PET files into an SFS