Rob has explicitly stated that SonicBlue are the only ones allowed to distribute empeg software. Which means that no one else can distribute a .upgrade file, as it contains their copyrighted software.

The best thing we can do is write a program that will take a SonicBlue-distributed .upgrade file, pop it open, insert new software, then stitch it back up.

However, if you want to make a new .upgrade file for your own benefit, go ahead.