The dc7100sff comes with 2 x SATA ports. So yes, you can fit a 2nd hdd to it. I don't know what "dual" connector you're talking about. Are you talking about the IDE cable? That's only for your CD/DVD drive. It is not for your hard drive(s).
The motherboard has 2 SATA ports. You may just need to buy a SATA cable to connect your new hard drive to it. The screws to mount the hard drive are found at the front of the case after you remove the cover.