Hi BP
I would have thought learning your own country's payroll would be a start. It might make mastering the concepts easier when you understand your own country's regulations relating to employee awards, etc.
As far as other countries go, that would then depend on who you work for. Are you in a consultancy providing support to a client or do you work for a multinational? If you are already SAP HR then I would look at the current system you support and if payroll is in scope try to learn there. It would be a more practical way to expand your skills.
Regards
Colleen