Yes,It's possible,
You will be able to use T-Mobile Sidekick in Ontario with Rogers plan.Before put sim card on your phone,Make sure your phone is unlocked one.If it's restricted by other than Rogers,you will need to unlock your phone.For unlocking you can approach your service provider or else try to buy an unlocking service from any one of the online unlocking service provider like Superunlockcodes.com Here you can unlock your phone at low cost.