Right click a part of a wall and there would be a choice to "Upgrade to Cornerpost", wich then costs the same as a cornerpost.
Right click a 2 or 3 section wall with cornerposts in both sides and there would be a choice to "Upgrade to Gate" and "Upgrade to Visitor Gate".
Yay or nay?
Also, don't know how easy or hard it would be to implement.