WP-SimpleViewer (the dedicated SimpleViewer plugin for WordPress) manages only galleries created by the plugin itself (and each gallery is linked to a particular page or post and cannot be reused).
You would need to embed your galleries in your WordPress site manually but this should not be too difficult or time consuming.
I would recommend using the baseURL method of embedding as documented here.
Essentially, to embed a SimpleViewer gallery into a WordPress page or post manually, you would upload the complete gallery folder (not just the contents) to your web server using an FTP program such as Filezilla and then paste the baseURL embedding code into the body of your page or post, ensuring that the method of entry is 'Text' rather than 'Visual'.
It does not matter where on your web server you upload your gallery folder to as long as the two paths within the baseURL embedding code (the path to the 'simpleviewer.js' file and the baseURL itself, pointing towards the gallery folder) are correct.
SimpleViewer Support Team