I don't know about official extensions of additional document deadlines, but I have heard from our MP that in general, the CIC is lenient on deadlines if they know that you are working towards submitting your additional document(s) in a timely manner.
In our case, they asked for a second police certificate within 45 days or have the entire application refused. It took us 15 days to get an answer from the Manila VO on what kind of police certificate they want (because they were not clear on that point) by which time we had 30 days. The Russian consulate says they take 2 months to process a police certificate, so considering we had to get it, have it translated and then sent to Manila, we would have been 45 days beyond the deadline.
Since our MP assured us that they will not automatically close our case on day 46, we weren't too worried, but to prove that we're working on it, we did send them extensive proof that we had applied for the second police certificate in a timely manner. (we sent a physical copy of the entire application for the police certificate, including copies of the application forms, copies of the notarized copies of my passport, copies of all the other documents we included in the application for police certificate, photos of the package, photos of the xpress post envelope, screenshot of the xpress post delivery confirmation, a tracking number for the return envelope, etc. We also sent them an email with a more condensed version of the aforementioned. )
Eventually we got the police certificate and sent it by Fedex. They got it 2 weeks ago, and of course no word from the Manila VO. I'm assuming no news is good news.