According to the official FAQ here, you can in theory, but there aren't any solutions available:
Can you use the FDE drives in an external eSATA enclosure and still use the encryption capability? In theory, the answer is yes. Seagate FDE drives and the underlying technology would lend themselves to the type of device envisioned here. However, at this time there are no eSATA solutions available to purchase.
In a related design, Seagate has also manufactured (but no longer manufactures) a line of external drive under the Maxtor brand called "BlackArmor". It uses a Seagate Full Disk Encryption hard drive within a USB enclosure. In addition, it has software to support the Seagate Secure features available in the drive. See here the Maxtor BlackArmor User Guide.
Of course, you could always use software-only solutions to accomplish the "external encrypted drive" goal, with Whole Disk Encryption software such as FileVault2 (bundled on Mac OS), BitLocker (bundled on Windows) or some of the TrueCrypt spin-offs, such as VeraCrypt (stable release available) or CipherShed (still on pre-alpha ATOW).