From my experience with Armory3D so far, it’s not exactly possible yet except for HTML builds of the game: Fullscreen or not fullscreen (html5)
However there does seem to be a way to program that into exported native builds using Visual Studio / platform specific applications: Armory most wanted? - #30 by sans-iccal
Best compromise I could find is exporting two versions - One that’s Fullscreen and the other that’s windowed. Course it’s not the best solution, but it works until that gets added into the source code. 