JA Comments Importer
The RSComments! - Ja Comments Importer will allow you to import comments from within JA Comments to RSComments!.
The plugin is installed via the default Joomla! installer. You can read more on how to install it here.
After the installation procedure is completed, remember to enable the plugin: Extensions > Plugin Manager > RSComments - JA Comments import : Enable.
To start the import procedure just head to Components > RSComments! > Import > Plugins and click on "Import" button.