An XPage is simply XML in a file with the extension .xsp, There is no trick here simply output the correct XML to a file. These tools would convert existing controls to XPages controls or where they can already be used, simply wrap them up in a container I would imagine.
The best approach however is to manually code these as you will loose a lot of the benefits of XPages by relying on these tools to create it for you. These tools are generally for large scale applications that would take far too long to rewrite or they are depending on legacy code etc.
A similar question was asked on how to do this and I answered it here:
How to convert forms to xpages programatically?