None of your "liberated" matter will come from the black hole.
The pair production mode you're describing comes from an interaction between photons and atomic nuclei. Essentially, when I high energy photon interacts with a nucleus, it has a chance of converting into a pair of particles, generally an electron and a positron.
You could, hypothetically, create some additional matter from high energy photons by doing this near a black hole and dumping all of the antiparticles into the black hole, preserving only the "regular" matter. However, doing so won't be liberating anything from the black hole, which is impossible. Black hole emissions are determined only by the mass of the black hole, as per the formula for Hawking radiation.
Instead, you're converting energy already outside the black hole (high energy photons) into another form of energy (elementary particles). Your conversion rate is 50% at best since you're getting rid of the anti-particles.
While it's certainly easier to build things out of particles than out of photons, you need to find a good way of concentrating your photons into a small space around a black hole, and then find a good way of getting your matter outside of the gravity well of the black hole. You also need to find a way of producing photons to feed into a black hole. All of this will net you a small amount of mass, since you're likely to be getting lots of high energy electrons out of your interactions, which are difficult to use as a construction material. Certainly not galaxies worth of mass.
At the end of the day, this is also probably something that's already happening at scales that would be difficult for you to replicate. Black holes, especially big black holes, tend to be surrounded by accretion disks of inspiralling matter. These disks are filled with lots of high energy particles compressing against one another and heating up. They certainly emit lots of high energy photons, and those photons likely interact with the particles in the disk to create particles through pair production. In really big black holes, lots of matter shoots out at the poles of the black hole in immense jets of high energy particles. Most of these particles are bits of the accretion disk that get launched free, but some are probably created by pair production. The amount of mass created, though, is far less than the amount of mass that falls into the black hole.
At the end of the day, yes, you can create mass through pair production near a black hole, but not very much, and it's not liberated from the black hole. You certainly can't create new galaxies this way, and are better off finding an existing galaxy and using that for whatever you need a galaxy for. There's certainly plenty to choose from.