You should differentiate between voxel *visuals* (i.e. blocky looks) and voxel *gameplay features* (i.e. block-based editable terrain and such). While they often come together, neither necessarily implies/forces the other.
If you want to create an RTS that just looks blocky, but has totally classic RTS mechanics, you don't need a voxel engine, and if you had one, it would only make a lot of things a lot harder. Just create your 3D models in voxel style and load/render them like any other with a "normal" 3D engine.
However, if you do want things like de- or con-struction of terrain, buildings or units based on voxels, you'll need a real voxel engine. In this case, you couldn't work with simple one-piece 3D meshes manually created with e.g. Blender. You'd need an underlying voxel data structure from which you'd have to dynamically generate your 3D models at runtime. This requires much more effort.
From a pure rendering point of view, voxel engines are still based on 3D triangle meshes just like "normal" 3D engines, since this is simply how (almost) all graphics APIs and GPUs work. The difference is the run-time creation of the meshes based on the underlying voxel structure. 3D models in "traditional" game engines are "hollow", "empty shells", so to say. There is no logic behind them which makes them look the way they do, other then the artist's creativity. The shapes of 3D models in voxel engines, on the other hand, are backed by the voxel system, which itself is "invisible", but defines spatial structure in the background.
I hope this helps 
