No, you can't use eSATA drives. There is no "slot" for you to add a eSATA card. (Current iMacs have a SD card reader, but there are no SD to eSATA adapters.)
You can use a [Firewire to eSATA adapter|http://eshop.macsales.com/item/Firmtek/SPYDERHUB>, if you really needed to. However if you're going to spend that much money, you may as well get a Firewire drive and get close to the same level of performance. (At least much better than any USB drive you'd attach to the iMac.) I'd go with the eSATA/FW adapter only if you have one or two eSATA drives whose total capacity is more than 1TB, since you can get 1TB drives for about the same price as the adapter.