Canon Rebel XTi User Question

Jeremy D. over in the forums posted a question that’s gone unanswered for a few days now.  Can anyone lend him a hand?

I have a Canon Rebel XTi (400D) and would like to buy an external flash. If I purchase the Sigma EF-500 DG ST, can I manually control the flash brightness with the in-camera FEC controls?

Here’s a link to the forum thread.