mojoPortal is designed to keep site content separated, but you can use IIS virtual directories to make the different site folders point to a single location.
ie you could copy and remove the existing media folder from under /Data/Sites/[SiteID]/media then for each site make a virtual directory in IIS at the same location but pointing to a different physical folder. in other words replace the physical media folder for each site with a virtual folder named media that points to a central location.