not sure if you can do this for just messages but if you go to people,contact that you want, and click ringtone you can edit them that way.
Note: if you have not set/trimmed a ringtone I do not think it would show up; only the default tones.
so, go to music, select your media, click set as ringtone. trim and click apply to contact ( not as default ringtone.) If you do not have music just mount your phone and copy paste you music into the music folder.