You can make an enormous lens where all the energy coming off a particular acre of moon goes through it. But you can't focus all of it onto a spot smaller than an acre, no matter what you do.
The current top comment here seems to agree with my original intuition, though.
This is the optical argument for maximum concentration given conservation of étendue (the same page has an optical argument for the conservation):
The angle subtended by the moon is approximately 0.54 degrees, or about 0.01 radians, so the maximum concentration factor is about 10,000. The moon provides about 0.1 lux of illumination, so the maximum illumination you can achieve by optics is about 1000 lux. The sun subtends about the same angle, and we receive 30,000-100,000 lux from it, and the maximum illumination you can achieve from concentrating it optically is about 1B lux.
I'm willing to believe that you can't light a fire from the moon, if the intensity's a million times lower.