How to upload movie from computer to "roll"/"galery" on iphone?

I have a problem with Imovie or Video Camera program.

On my computer I prepare "intro movie" for my movies. I want to put it via imovie, so I could integrate: 1. My movie from computer + 2. My movie from iphone.

The problem is, that 1-my movie from computer is "video" on iphone AND 2-my movie from iphone is in "roll galery" 😟

Can you help me?