Hi,
If the module name is payxpert, in /modules/payxpert/payxpert.php (rule is /modules/module_name/module_name.php), you will find the method which starts with:
public function hookDisplayPayment($params) { ...
You can copy the entire function and rename it to
public function hookDisplayPaymentEU($params) { ...
Once you've done that, you'll need to add it to Hooks, it will become available for hook register.