You could also get a Sustainiac. In fact, that's probably the best bet because its less intrusive to fit than the Fernandes and only requires one 9v battery, I think. You can also use a Sustainiac with any bridge pickup.
Sustainers work together with the bridge pickup. So when the sustainer is on, the bridge pickup is on too. When it's acting as a sustainer, it's not a pickup. It uses a magnetic field to create sustain/ harmonics, which your bridge pickup then amplifies.
I think they're very cool, but I like the sound of my existing neck pickups way too much to put one in any of my current guitars.