setup a network of sites

Hi – i have a newspaper type site for town A. It is doing well and I want to expand to other towns and setup a network using multisite. I’d like to establish a new domain as the “hub” and then use the existing site townA.com and make other sitestownB.com, townC.com, etc and add them to the network. I am a newbie to multisite, btw. My question is, am I doing this backwards because I have a website populated with content but essentially I want it to be NOT the master site (or “hub”) but the sub-site. Would I just get the master “hub” domain, setup wordpress multi-site and a new theme and then somehow link townA.com? How would that work?
thanks in advance for any suggestions!! lisa