thats a good question, i know purolator has just released a new shipping mod but im not too sure how you would integrate it, but they do have a website with lots of info, if you have the time to do your research i suggest you check out eship.purolator.com i hope this helps you get your answer, if i come accross anything ill post back.
Purolator does have a nice set of web services, so creating a Purolator module would be a reasonable thing to do. But it's only going to get written if it's important enough to you that you're willing to write the module yourself or hire someone to write it for you.