You can't really convert .pet files to .2fs, even though both files originate in Pupply Linux. First is a installation package, the second is used to save a current version of the system "a snapshot". No way to convert one to another.
Convert Puppy Linux install package file to Puppy Linux snapshot file.
We found one software available in the database for this conversion.
Linux/Unix software - convert pet to 2fs on Linux
A lightweight distribution of Linux
Convert from pet 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.