In theory, you can create a map in Blender and then import it into Radiant, as a model (or a collection of models). Or you can use a Blender export script to directly export the map to a .map file. Game entities are then added in Radiant.
However, several mappers have tried this (including me), and I didn't see any convincing attempt yet. There are lots of issues, concerning lightmaps, collision surfaces, vis, performance...
It seems that the best way to go is to build the base structure in Radiant, and then create models in Blender that are placed in the map to enhance it visually. If you are able to work with Blender, I really don't see how you couldn't learn how to work with Radiant.