If you are building a POI that the game is going to spawn as a POI then you use the filler blocks in creative mode. If this is a player blueprint then you don't.
That's a shame. I wish they'd add the ability to add grass, stone, boulders, waterfalls, and flora to the actual bases. I really wish we had some more flora and decorative plants. The base I'm trying to build, I envisioned putting a large garden with a pool in the middle of a Courtyard setting
