Contact them, ask them. Go to their site, I browsed a little, didn't find much, but all of their artists had kits way bigger than the pre-set config's that they offer. I'm fairly sure that they would offer you an add-on tom. Just drop them an email, and ask.
i'm almost 100% positive that if you contact them, they can get you an add on. Ddrum started out as a custom drum company, then they hit it big, and started making models and more affordable stuff. they are STILL a custom drum company though, and i don't know what custom company out their couldn't get you an extra tom if you wanted, if they couldn't then they wouldn't be a custom drum company.