If you mean edit/modifying Silverpine's layout to add in a building, so your custom NPC has a place to stay, then at the moment, no.
If you mean, using your player character to construct a building on some NPC's plot of land for your custom NPC, then yes. But you'll need to have that NPC's key to build on their land, and a whole bunch of gold, materials, and time to build and furnish it all.