It doesn't seem possible to directly convert CATPRODUCT assembly files to STL format. However you can produce CATPART files for individual parts from the assembly files and these could be exported as STL. In CATIA and other CADprograms with its format supports there usually function for producing CATPARTs files out of CATPRODUCT.
Some dedicated CAD converters can also automate this process and directly convert CATPRODUCT to STL. This will most likely create separate file for each part in the assembly.