I don't think you can. The Sidekick doesn't use T Mobile's servers for texting and internet, it uses Danger's which is routed through T-Mobile and proprietary. So, in other words, like the iPhone, you have to use the respective provider. In this case T-Mobile, which doesn't exist in Canada (I don't think).
What are you looking to do with your phone? The Sidekick is kind of old technology and there are many phones out there that are much better.