Depending on your account and credit you may be able to add another line to your account but it needs it's own plan. Keep in mind that calls and texts between two phones on the same account are free, good to know when you're choosing your plan.
You can use tabs for each of the new phones so depending on what phone you want, it could cost nothing out of pocket.
Chad is right, depending on your credit class you can have one or more lines on your account, each with its own plan and Tab. You can find our current plan offer here http://koo.do/qqOCIX (Canada-wide family calling, unlimited Evenings & Weekends included). We currently have some great phone deals too, check them out here http://koo.do/kphone.