I don't think there's an easy way to do this. I'd be looking at reading the file using tFileInputFullRow and then in tJavaRow, splitting the fields to an array and then looping through the results putting them back together in groups but with a different separator between the groups so that you can then use tNormalize to put each group on a separate row.
You could try the Talend Exchange component tSchemaNormalize.
edit: on second thought, adding rows when the flex is a part of a flow is probably not possible. use a tMap to multiply and then a flex to process.
I have no clue, what prevents you from using my suggestion. The mentioned component tNormalizeSchema does EXACTLY what you need.
This component has a PDF documentation and if you would take a look into the examples, you would find exactly your use case.