I think you can try something like this, put it in your theme's functions.php
add_filter('woocommerce_paypal_args', 'convert_rmb_to_hkd');
function convert_rmb_to_hkd($paypal_args){
if ( $paypal_args['currency_code'] == 'RMB'){
$convert_rate = 0.8; //set the converting rate
$paypal_args['currency_code'] = 'HKD'; //change RMB to HKD
$paypal_args['amount_1'] = round( $paypal_args['amount_1'] / $convert_rate, 2);
}
return $paypal_args;
}