If you want to export the strings from a .po file, you can use the Customisable Export option.  You can select all of the source strings for export, but this gives an error message.  The message is wrong.  It says this:

>>>One or more of the selected translation lists are already exported.

RWS support tell me that there's enough information at this point because there's a comment against the selection that says this:

>>>This format cannot export source lists.

I don't find that comment helpful - what is 'This format'?  RWS support tell me it refers to the .csv format.  I don't think that is obvious.

I think Passolo should not allow you to select a source list for export.  It should give a clear message at that point to say this:

>>>You can't export source lists for .po files.  Try exporting a translated list instead.

(It would be nice if RWS support passed on concerns to development, instead of making you raise them again in another forum.)