I don't think you have to add them. Carbs will show your TOTAL carb count including sugar and fiber.
I think MFP has them separate because it's more common for simple "CICO dieters" to only be limiting sugar and not carbs. Even many diabetics are instructed to only limit sugar as if somehow other carbs don't have the same effect.
Anyway, on low carb you'll already be limiting sugar so you can ignore that for the most part. Unless you start to notice that more of your carb allowance is made up of sugar than anything else then you can decide if that's an issue for you.

As noted above, total carbs include sugar and in the US they also include fiber (fiber is excluded from total carbs in the UK).

As a diabetic, I find it helpful to assess my total carb count and the total sugar count as sugar will affect my BG much faster and with a much greater impact than other types of carbohydrates. Whilst I generally eat 25-35g carbs each day, I couldn't eat that much actual sugar without my BG going through the roof, so I try to keep my sugar under 8g/day. I try to eat a pretty clean diet, so most of my carbs come from vegetables, but I do use convenience foods when I'm busy, such as salad dressing and sriracha, so those are the type of products that I have to really watch to make sure my carbs aren't getting too high and to make sure that those carbs aren't all sugar. Technically if someone was of the philosophy that they're happy to eat whatever they want if it fits their macros, at 25 g carbs each day, they could put a couple tbsps of sugar in their coffee if they had no other carbs, but that wouldn't work for me because my BG would get too high.

Foamroller wrote: »

I only wish it could become known in the general public that the body doesn't care whether you're eating sugar, wholegrain bread or pasta...ALL starches sans indigestible fiber turns into glucose anyway. The only difference is the time it takes for the body to break down the molecules.

I wonder if this quite arbitrary division, metabolically speaking, between sugar and starch is meant to confuse rather than enlighten. If I see another "healthy" muffin in the high end coffee shops, I'm gonna scream

I would be happy if we could just get the doctors who treat diabetics to understand this. It is infuriating to see the diets recommended by doctors to diabetic patients.