Air France operates a bus service with 2 routes to central paris. One goes to Gare de Lyon and Gare Montparnasse the other to Paris Maillot and Place de l'Etoille. From any of those locations you can connect with the Metro. But watch out on some stations, they're notorious for pickpockets. See a list here: http://safety.worldnomads.com/France/73442/Crime-in-France
Air France also have the Super Shuttle which will take you door to door... for a price. over 6 years ago
I travelled *out* to Charles de Gaulle airport from the Gare Du Nord train station... very easy. Bought metro tickets, hopped on the Blue line (RER Blue) and got off at the other end. I imagine it's just as easy in reverse.
I don't know the 9th Arr, but my experience of the Paris Metro was that transfers between lines were easy, and the stations easy enough to get around.
I found this useful page: http://parisbytrain.com/rer-train-from-paris-to-roissy-charles-de-gaulle-cdg-airport/ Particularly the link to the RER map at the bottom over 6 years ago