Navigo cards are just cards. Even if you had one in advance, and I know of no way to do that other than what Landing Gear has suggested above, you must still load a fare onto the card before you can use it. If you want a Navigo Easy loaded with tickets t+, that could be done in advance by a concierge as the Easy is not user specific and the t+ is valid until used.

However, before you can use a Navigo Découverte, it must have a 25 mm x 30 mm photo (passport like in black and white or color) attached, your name legibly printed/written in the space provided, and an appropriate fare loaded. Weekly fares are time sensitive and can only be loaded and used for the current Monday through Sunday week up to and including Thursday. Fares loaded on Friday, Saturday, or Sunday are not usable before the following Monday.

The easiest way to use the Navigo Découverte is to just purchase, configure, and load a fare when you need it.