Say I have an object that uses multiple textures for the one object, but I want there to only be one texture. Is there a way I can unwrap a UV map into an image?
Depends. If by multiple textures you mean a diffuse texture, a roughness texture, etc, then no. A single image can only handle 4 channels of data, which a diffuse + alpha takes up all on its own. You can’t pack more onto a single image (although you could pack roughness into one channel of an image and metallicity into another channel, for example.)
But if you mean, you’re using some mask to mix between different images on different parts of your mesh, for a single property (like diffuse color), and you want to write all that into a single diffuse color image, then yes, you can do that.
What you’re looking for is the concept of texture baking. Quick way to get started is to just do it. Change to Cycles engine if necessary, add an image texture node to your material, create an initialize a new image for that image texture node, and leave that image texture node as the active selection in your material. Select the UV map you want to use for the bake for the mesh being baked. Make sure you have a light in your scene. Go to properties/render/bake and bake combined.
After doing that, you can start playing with the bake settings to get what you want. Remember that you can always edit materials temporarily to get them to bake what you want them to bake.