Cubemaps Lerp

Hi, I need to smoothly transit from Day Cubemap to Night Cubemap over time. But in my setup I don't have direct access to the shader that's using these cubemaps. So I also don't have access to this shader blending. All I can do is assign a single cubemap to this object. Can I somehow create a cubemap that's lerping between other two cubemaps? I'm kinda newb for this whole shader thing so I hope you get what I'm trying to achieve. Thanks.