This is all quite achievable though code, when your creep is searching for / selecting a construction site (or the manager you've created to assign goals to creeps) you can prioritize/filter/arrange the data in any manner that you'd like. I suppose, feature wise UI you 'could' do it where it reassigns the Game.constructionsites object for the selected construction site to be first in the array, but not sure how feasible that is, and that may not work for everyone.
Some quick examples, is when your builder creep is selecting/getting assigned a construction site, you can first check to see if any towers or specific types of buildings are in the array/object then if they are, use that element and ignore the rest, if not, then proceed with whatever.
I'd highly recommend checking out the World:Help section of the Screep's Discord, folks are normally very helpful and its a bit more dynamic/faster than the forums.