I've seen around some custom-made btc addresses, for example
1Payday1y58nDwp1t......
notice the "Payday", some pools have that word in the address from which they send you the withdrawal
how did they choose that?
i'm not savvy to the inner workings of btc, but when you generate a new address i thought it's choosen at random by the network.
maybe you can "ask the network" if a specific address is already in use, and if not you can create it
They generate a random address. Then they look to see if that random address starts with the letters they want. If it doesn't, then they throw out the address and repeat the process. They run a computer program that does this as fast as possible and try millions or billions of random addresses until they stumble across one that starts the way they want it to.
For more information, search for VanityGen