They way I do it is to import the file into a global field as a reference. I can then parse the path from the text stored in the global field.
This is one way to create the path:
Let ( [
~importedFile = Substitute ( LeftValues ( Table::GLOBAL.FIELD ; 1 ) ; "file:" ; "" );
~exportedFile = "myFile.xls"
Substitute ( RightValues ( Table::GLOBAL.FIELD ; 1 ) ; ~importedFile ; ~exportedFile )
Set it to a variable before the export step and the use that variable the specify output file dialog box. The ~exportedFile value is literal in the example, however normally I would use a variable or calculation to dynamically name it.