I remember that if you order through the HTM website, one of the options for the Assisted Torpedo Gunhammer is to add serrations (for $5 extra). I just looked at the page for the Assisted Bowie Gunhammer, and strangely there isn't an option for serrations. But given that they offer the option of serrations on the Assisted Torpedo Gunhammer and on all of the Auto Gunhammer variants, I'd be very surprised if they weren't able to do it for you on an Assisted Bowie Gunhammer.
Give HTM a call at the number on their website and I'm sure they'll take care of you. :thumbup: