iPhone does not support directly copying of iPhone contacts to the SIM memory. Easiest would be to
1.[http://www.copytrans.net/iphone-contacts.php|Transfer iPhone contacts] to PC
2. Use a third-party SIM manager program in combination with an external USB SIM-card drive for your PC in order to save the iPhone contacts from your PC to the SIM card.
3. You are all set!
Having a jailbroken iPhone complicates matters, as you cannot use iTunes to sync your iPhone contacts to Outlook or Windows contacts in order to transfer the contacts in both steps 1. and 2. you need to use third-party programs.