In the Unreal Engine, there is a concept known as level streaming, where you enter a volume, and the scene behind you is unloaded, and the scene in front of you loads, creating a seamless transition from one level to the next. Examples of this on a small scale would be like Bioshock or Mass Effect, where the former used an underwater walkway that went between massive structures, and the latter used elevators, much to the players' chagrin. Bethesda uses a similar method, but not as obvious, using tons of LoDs, and each chunk streams(essentially) the 8 chunks around it, having Radiant run in the background for NPC logic.
Like @syclamoth said Application.LoadLevelAdditiveAsync is exactly what you want but like you already figured out it's a pro-only feature. In the free version you can use Application.LoadLevelAdditive, but it's not that "seemless" since it will block until it's loaded.
I wouldn't say that it isn't possible in the free version. You might want to use you own scene format (some kind of xml scene). You have to find your own way to load (stream) all kind of assets (mesh, texture, material, gameobjects). It's still probably not as seemless as with LoadLevelAdditiveAsync but it should work. Note that some things can't be created / accessed from scripting but just a few (for example only ParticleEmitter is exposed to scripting but not the EllipsoidParticleEmitter so it can't be created via code). A workaround is to create some simple prefabs which can be instantiated.
I haven't done this before, but you should do some testing before you decide to go along with a specific method.
answered Apr 29 '12 at 11:13 AM