Recreate basic AVCHD from MTS files
Guess the original poster found his way to do it - I found the following that worked for me, even though I followed a different procedure. I didn't want to convert the MTS files before importing them to iMovie into anything else and was missing too the additional index / descripition files. this is the source and works for Sony SW / Cams. Potentially with other HW / SW constellation a similar way is possible.
http://www.mac-forums.com/forums/movies-video/263323-how-import-mts-files-into-imovie-have-been-archived.html

So what I did:
1. I added the folder with my MTS files on my USB drive with the Sony PMB Picture Motion Browser - in fact I used the more recent Play Memories Home edition
2. Now I marked the MTS files from that source and exported them to an external USB drive - but you can use basically any medium (SDHC, maybe even DVD if you think that's better for archiving)

And now the new folders created can be either imported directly from the medium, copied to the Mac or other location from where you again can import them as archive in iMovie.

Good luck.