Does modularity mean we will be able to install a scanner station/extra fuel tank/computers/medbay? Is there something we can install into the gap in the middle?

Please see the above answer. The negative space is not a hardpoint, and plumbing or piping it to serve as one would impose an added cost in mass that doesn't serve the Hammerhead's military mission profile. The Hammerhead, though, provides an excellent defense for ships that you'd want to carry scanners, fuel, or medbays, and most would want the Hammerhead's existing computer and scanning resources devoted to enhancing the performance of its weapons first and foremost.