Is it okay to add extra sugar to a specific brioche recipe?
First of all, this recipe is not supposed to taste sweet. There is so little sugar, it must be there for tenderness - and even this is strange in a bread with so much glutenless filler (the potato). If it is in the sponge only, maybe it is only there to speed up the sponge rising time. 

Adding sugar to a bread recipe is not trivial. Sugar interferes with gluten creation. The maximum sugar you can have in a bread recipe is two tablespoons sugar per cup of flour, which is not very sweet. And then you have to change the recipe: it needs much longer kneading, and more yeast. At very low concentrations, sugar does indeed feed the yeast, but if you use more of it, its osmotic activity interferes with its growth. 

The probelm here is that you have all the potato in the recipe. This means you already have very little gluten compared to the mass of dough. And  also that it is diluting any sugar, reducing the sweet taste. 

You can certainly try adding up to 7 tablespoons of sugar, but you should knead longer (double the knead time for the full 7 tablespoons - I knead standard brioche for about 30 minutes per hand before adding the butter) and will have to live with a bit worse texture. You can also try to use gluten strengthening tricks to compensate - add ascorbic acid (vit. C powder or a crushed pill), start with cold water, or add vital gluten.