There are some very rudimentary tools inside UE4 which allow you to create some basic meshes (models). The thing is, that it will be slower and more painful process to do it in UE4.
I would understand reluctance to get into some modeling package if the money was the issue, but with Blender it’s not the case, since it’s free. I mean, even inside UE4 itself, you will probably have to spend at least couple of days learning how to use UE4’s modeling tools to produce some models. You can instead use that same amount of time to learn to do that in Blender, and since Blender is built to do exactly that, then after those couple of days spent, you will be a lot more productive in Blender than in UE4 when it comes to authoring 3D models.
If all you have is ideas and script for your game, and you want to pull off a miracle of actually finishing it (which 95%+ people give up, after they realize how much work is involved), then by insisting to stay just inside the engine and not using any external modeling tools, you are intentionally reducing your already quite small chances 