Give it a shot then. eSATA and SATA are pretty similar in terms of Architecture. Have a look at the wikipedia entry on eSATA, regarding it's differences from regular SATA:
The external cable connector equates to a shielded version of the connector specified in SATA 1.0a with these basic differences:
* The external connector has no "L" shaped key, and the guide features are vertically offset and reduced in size. This prevents the use of unshielded internal cables in external applications and vice-versa.
* To prevent ESD damage, the design increased insertion depth from 5 mm to 6.6 mm and the contacts are mounted farther back in both the receptacle and plug.
* To provide EMI protection and meet FCC and CE emission requirements, the cable has an extra layer of shielding, and the connectors have metal contact-points.
* The connector shield has springs as retention features built in on both the top and bottom surfaces.
* The external connector and cable have a design-life of over five thousand insertions and removals, while the internal connector is only specified to withstand fifty.
Looks to me as though plugging the eSATA case connector into a SATA port on your motherboard should work. You shouldn't need to specify anything special in BIOS, just make sure that it's set to boot off of an internal SATA hard drive. Try it out and post back.