add a phone number same account ？

Hi Hoc

Sure you can, probably will more interesting a new account and using your referral code (after 2 months you will have 25$ credit on your first account and on the 2 account)

This is the full guide
https://fizz.ca/en/faq/how-do-i-add-plan-to-my-account

How to add a plan to your account:

1- Go to Overview in your Fizz account.
2- Click on Add a plan.
3- On the next screen, click on Order a new SIM card (if you don’t already have one) or buy from couche tard https://fizz.ca/en/sim-card
4- Order and pay for a SIM card.
5- When your SIM card arrives, Go to Activate my SIM Card. https://fizz.ca/faq/activate
6- Enter the activation code on your SIM card in the field provided.
7- Follow the process for creating a plan

Yes you can, but as Mike suggested why not open another account and use your referral code to refer yourself? You make a total of 50$ (25 each)