The only way to do that would be through the Creation Kit. There's no safe way to be able to do it in real-time in-game because there's the risk and high possibility of adding cells to the territory that fall within the build perimeter of the expanded settlement but actually don't belong to the settlement, causing confusion to the game and a high probability the settlement wil get turned into a 'Commonwealth' entry and lose some its functionalities. For the vanilla settlements, like jjb54 already mentioned, the Build High mod is your best option. It expands all settlements (except Home Plate -which isn't a settlement anyway). It also makes the WorkShops of Homeplate, Boston Airport and the Mechanist's Lair into full operational settlements with all build options available. the vanilla settlements are the least of my concerns, all settlements expanded mod works okay for them, with a few exceptions. but i have a few mdod added settlements that i like location wise but hate when it comes to the building area (some red rockets turned to setttlements where the build area is basically the roof of the station