JuiceboxBuilder is unable to directly convert a SimpleViewer gallery created by Porta as Porta uses an older version of SimpleViewer (v1.9) before SimpleViewer was rewritten from scratch for v2.0.
Also, the galleries created by Porta use non-standard names for image folders and non-standard tag names within the XML file.
The only way I have been able to convert a SimpleViewer gallery created by Porta is a 3-step process.
(1) Open up the 'gallery.xml' file and replace all instances of <name> to <filename> and all instances of </name> to </filename> (so that svBuilder recognises the tag names).
(2) Open and save the gallery using svBuilder (to update the gallery from v1.9 to v2.3.1).
(3) Open and save the gallery using JuiceboxBuilder.
If you have a text editor such as Notepad++, you can open all your 'gallery.xml' files at the same time and change the names of all the <name> and </name> tags in two global search and replace actions (one for each tag).
Then, it should not take long to quickly open and save the galleries first using svBuilder and then JuiceboxBuilder.