In theory it oughtn't to be too hard to figure out the numbers re: length and measure for a music track.
Basically, just listen to the music carefully, counting time to determine how many bars there are in the selection (e.g. if you count 256 beats and the piece is in 4/4 time, then you know there are 64 bars in the selection). This gives you your first number. Then open the wave file itself in your sound program and find the information on how many samples there are in the thing. Take this number and divide it by the number of measures, and then divide that again by 2, and that will give you the number of samples per measure, which is your second number.
The reason for the division by 2 bit is that that way FS2 will switch to a different wav file on a downbeat (i.e. beats 2 or 4), rather than just any arbitrary place (which would almost certainly be in the middle of a note, which would sound terrible). Also note that this would indicate that FS2 assumes all the music to be in 4/4 time. If you wish to add a piece in, say, 3/4 time, things will get far more complicated, but not impossible.
I must stress, however, that this is theoretical. I haven't actually tried doing this myself. I merely have the knowledge.
