As far as I understand it, you can mine an account for somebody else. The payment will be done with a specific type of token, different from Steem and that token should belong to somebody.

In the early days, you could mine your account by editing the config file of your steemd instance, and setting the name of the miner with your desired username but you had to previously verify if that username is not taken on the Steem blockchain. I don't know if this will be available in the new implementation, but I'll keep you posted.