Yeah I'll help you out here, I am under a similar circumstance right now and from what I have observed you can certainly start an application online through "Apply to come to canada" in your ircc login page.
Then there will be questionarie and somewhere in that questionaire it will inquire what status do you have right now? And then you will say "Student" - this will generate a list of required documents for study permit extension and here is where you will see a big form which opens in a verified pdf in adobe and in that form there is a section which asks specifically when have you entered canada. Now since you haven't - the form will have a shiny red block on that left block and you won't be able to click the validate button to save the details and submit it in the ircc portal, so you in the end won't be able to complete the application.
So tldr; you can start your form, fill in the other details - upload documents like DLI number, new offer letter, financial proof and all that, but you can only complete the form from within Canada and then finally submit it. But I must say that please remember to not to start the application too soon from outside, as the portal only allows you to keep the ongoing application in the system for 60 days - so my recommendation would be you start about 15 days before leaving for canada.
Hope this helps! If you've got any other question lemme know.