Have the project open you want to export as a file. You want to Export Using QuickTime. Then you will be able to export it as pretty much anything you want. Apple Intermediate Codec is what iMovie transcodes AVCHD in to, as you can keep it as that, or you can make it ProRes, XDCAM, DVCPRO HD, RAW, H264... so many options when you choose that setting. I would lean toward ProRes or Apple Intermediate. Just be sure to have your frame rate set as native or same as source, dimensions as 1920x1080 HD, Aspect Ratio as 16x9, and audio as uncompressed Big Endian. So many manual settings to go through when you do it this way.