Check out Audacity for converting to .ogg format.
As for adding to the soundmusic.sounddata file, you add to it same as to any other list file in SoaSe:
-sound files go into the Sound folder of your mod
-append the new entries at the end of the soundmusic.sounddata file
-make sure you increase the count number at the top of the file to match the new number of entries
-make sure you leave at least one blank line at the end of the .soundata file
As for how the new entries should be formatted, simply copy/paste the existing entries and change the relevant information, in this case the music file name and make sure you have the emotion level and emotion range settings matched with the musical theme (action, peaceful, etc.) - best bet is to examine how the existing battle, trouble, quiet etc. themes are set up.
Good luck!