It looked as if the remaining part is still there. A bolt extractor "should" do the trick.
Also, make sure you buy the OEM replacement - just like the one you broke. The hole in the shaft is made so you can still drain the oil if it breaks, but also to "easily extract the remainder". The bolt itself is made to shear if too much torque is applied to the bolt, and thus to save the thread of the oil pan, which it was mentioned earlier is a pain to remove on the i6.
Looking for a DIY? Parts? Check this out, it might be your ticket
TMS underdrive pullies - Stewart WP - PSS9 - Beisan Vanos seals - Zimmerman cross-drilled & Akebono Euro - Deka 649 MF - 55w HID headlights - 35w HID foglights - Hualigan double din - ACS (rep) alu pedals - Euro central storage console - Breyton Magic Racing staggered wheels - M5 bumper - M5 steering wheel - Tint
Stable: e39 M54, e53 N62 & Tribby