Well, you have a few options -
- If you want them all as individual clips, then you can export each one individually
- If you want them chained together in a movie of sorts, you can create a project in iMovie and in that project layout the clips in the order you want them to and export that.
- If you import them all, but don't want to use iMovies conversion, you can reveal in finder the master footage on your computer and from there you can use Handbrake, mpegstreamclip, etc. to convert the footage to a different container/codec for viewing (when iMovie imports HD video, it will store it using the AIC codec which takes a good amount of space).
Of course, if you just want one of the .mts files, as I mentioned above - you could just convert the individual files you need (one thing to consider is if the recording you're converting is longer then what can be stored in a single file, multiple .mts files may be one event recording) - you can also use VLC or movist to play the raw mts files if you want to see them without using iMovie.