Why not?
because it will ferment out and then you'll have to rack again etc. if you want to increase the abv by adding sugar, you can do it right in the beginning.
Half the learning part is to know why you are doing something. Why do you want to add sugar to the secondary? So you want more alchohol? Trying to make it sweeter? Are you adding it as a primer getting ready to bottle? The why part is important.
And yes you of course can add sugar or anything else you want in the secondary, we sometimes top off with sugar syrup when going from the primary to the secondary if we didnt have enough volume to fill up the carboy. Its all in the WHY part.
WVMJ