There are adapters for eSATA to USB, but they are actually the reverse of what you need. They are designed to plug an eSATA hard drive into a USB port. They will not work the other way around. As mdram said, the best thing is to get an external hard drive enclosure that has an eSATA port, and then get a huge hard drive to put into it. You can go up to 2TB. The 2.5 and 3TB drives are out now, but you can not use them because they require special configuration which the HR24 won't support. Go as big as you can afford, because the external drive REPLACES the internal drive - it doesn't add to it. So adding a 1TB drive will double your capacity, not triple as you would expect.
Oh and just a couple other tidbits.. Either put the external drive out of sight, or cut the wire to the activity LED when you are assembling it. It blinks constantly even when watching live TV (using the buffer), and it's obnoxious if it's anywhere in your field of vision. Also, if you can go physically look at the enclosure at best buy or micro center, check to see that the power connector locks in place. If the power gets cut off while the drive is being accessed, you can lose data or corrupt the drive. You don't want to lose your whole drive because you were reaching behind for some reason and knocked the power connector out.