Hi,
I'm not sure about the requirement to change a standard format. Please check:
https://en.wikipedia.org/wiki/Resident_registration_number
The format has to be XXXXXX-XXXXXXX.
Anyways, technically, there isn't much you can do to change it. SAP expects the number in the format and throws out an error if it does not match. This is done in method CHECK_REGNO in class CL_HRPA_KR_CHECK_REGION_NUMBER. You can also refer to OSS note 1164333.
Hope this helps.
Donnie