I will get a Navigo Weekly Pass (Zone 1-5) and am wondering if it can take me to Chateau de Chantilly. It looks like I'll have to travel either by a TER train or RER D train to Creil from Gare du Nord (both stopping at Chantilly - Gouvieux) which seems to be outside Zone 5. Does this mean I have to pay extra for the part of journey from Zone 5 to Chantilly - Gouvieux, or I cannot use the weekly pass at all?
For Gare du Nord to Chantilly (Gouvieux) you'll need to buy a ticket for the portion outside of zone 5 (Survilliers Fosses to Gouvieux for RER D). Unfortunately I don't know of a system that works smoothly for that. Origin-destination RER tickets can't be added onto Navigo cards so you'll need a physical ticket for that portion. And the ticket can't be validated at Gare du Nord so you'd need to exit the train at Survilliers Fosses, exit turnstiles/gates and re-enter to validate your ticket and get the next train.
For Ter trains, I don't know what the supplementary ticket would be since the only stop between Gare du Nord and Gouvieux is already outside zone 5 (Orry la Ville). I would imagine you'd just have to buy entire trip tickets for Ter C11 / C13 trains for Gare du Nord to Gouvieux and skip using your Navigo entirely for that. The Ter train is significantly faster and less stops so that would probably be my recommendation.