There are some sugar free hairball treatments out there but they are hard to find, expensive and messy to use unless you put it in their food.
Funny story, I was at Walmart and saw a bunch of Sugar Free Hairball Gel on the clearance rack and yelled out loud "OMG I can't believe it" once everyone was staring at me I grabbed the tube, held it up and said "It's sugar free hairball gel" I guess you have to have a diabetic cat to appreciate that because all I got was blank stares. :shock: