If you need to create a custom conversion specification, use a predefined conversion as a starting point.
Click to copy to the clipboard the w2x command-line options which are equivalent to the currently selected predefined conversion. Then paste the contents of the clipboard into the new text file which will be your custom conversion specification.
For example, clicking while selected predefined conversion is "DocBook v5.0 article" combobox copies:
-o docbook5 -p convert.set-column-number yes -p transform.cals-tables yes -p transform.hierarchy-name article
to the clipboard.