How can I add more alternate links for current page?

You can use 'bwsplgns_mltlngg_add_alt_links' filter to do that. For example, you want that search engines firstly indexed exactly English-language pages of your site for people from New Zealand and South Africa. Complete the following: 

1. Go to the plugin settings page and switch to "Custom Code" tab;

2. Mark "Activate custom PHP code" checkbox in the "PHP" section;

3. Add the following code: 

function get_additional_links( $links_attr ) {
/* English for New Zealand */
$links_attr[] = array( 'hreflang' => 'en_NZ',
'link_param' => 'en_GB' );
/* English for South Africa */
$links_attr[] = array( 'hreflang' => 'en_ZA',
'link_param' => 'en_GB' );
return $links_attr;
}
add_filter( 'bwsplgns_mltlngg_add_alt_links',
'get_additional_links' );