Where to add custom currency?

You could use the em_get_currencies filter to add a new currency. For example if you wanted to add the Ukrainian Hryvna you could use the following code snippet:

add_filter('em_get_currencies', function( $currencies ) {
    $currencies->names['UKR'] = 'UKR - Ukraine Hryvna';
    $currencies->symbols['UKR'] = '₴';
    $currencies->true_symbols['UKR'] = '₴";
} );

You can use the Code Snippets plugin to add this code snippet.

You can see the existing supported currencies in the file wp-content/plugins/events-manager/em-functions.php in the function em_get_currencies.

Yes, it will be overwritten every time there’s an update. Here’s a corrected version of the code snippet:

add_filter('em_get_currencies', function( $currencies ) {
    $currencies->names['UKR'] = 'UKR - Ukraine Hryvna';
    $currencies->symbols['UKR'] = '&#8372';
    $currencies->true_symbols['UKR'] = '₴';
    return $currencies;
} );