Do you mean add a new line? You would need to sign up for that first, and then once approved, contact Koodo and ask them to activate your current (unused) SIM card with that.

No. I moved to Shaw Mobile a few months ago. I drive for skip and door dash and the service is terrible so I want to use my old phone with a new sim card. When I log into Koodo, it recognizes me but won't let me do anything cause the screen freezes and says pick an account but there is nothing to select and no way to exit out

If you cancelled the account, you can't use that account for any purpose other than checking your past bill after 90days of cancellation.

Regarding the phone and SIM. You can put your Shaw SIM into Koodo phone. If you want to get a different SIM card or phone from Koodo, you would have to register again as a new customer.

It's a brand new never used Koodo sim

As I mentioned,you can't activate your old account any longer. if you wanted to activate that new SIM, you would have to activate a new account online/kiosk or contact Koodo directly. Connection fee of $50 might apply if you do it in store or ask a rep to help.

You could also look into Koodo prepaid or Public mobile plans. Those plans didn't have certain features of Koodo postpaid but might satisfy your current need.

To clarify, koodo does not have a system to activate or re-activate customers with a random unused sim.

Accounts are activated via online sign up (and they send you a sim they activate with no option to activate the new sim you currently have with you) or instore (connect fee applies)