Workout files are located in the WORKOUT folder. You can create/edit one with any text editor. A workout file consists of any number of lines. A line contains two values delimited by a comma. The first value is the heart rate zone, the second is the number of minutes for that zone. An example:
1,4 2,3 3,2 4,2 5,1 4,2 5,1
This is a short 15-minute workout. You will start in zone 1 for 4 minutes, followed by 3 minutes in zone 2, then 2 minutes in zone 3 and then 2 minutes in zone 4. Then it's 1 minute in zone 5 followed by a recovery of 2 minutes in zone 4. The last minute will be in zone 5.