Adding a new company code to the existing SAP system

Use EC01 to copy an existing company code. This will ensure that you will not missed out any customizing settings.

OKKP - Assign company code to the controlling area
OBY6 - Define company code golbal parameters
OB62 - Assign company code to chart of accounts

Then check the following if needed, there may be other areas depending on your needs:

OBA4 - FI Tolerance Group for Users
FBZP - Maintain Payment program
OBYA - Inter Company code clearing if being used