Right - the SSG-*M devices are essentially J-series platforms. http://kb.juniper.net/IKB14032 has details
Our Migration Guide - http://www.juniper.net/techpubs/software/junos-security/junos-security10.4/junos-jseries-migration-guide/junos-jseries-migration-guide.pdf contains details on migrating using a USB source.
I think there's a bit of a catch-22 to boot from USB - I believe you must first prepare the image...using Junos. So if you have another Junos device, maybe you can do it that way. Note that to obtain the Junos download role you have to show that you own a Junos device (which buying the conversion kit - see below - provides)
Google or KB search on "Migrating to JUNOS Software with Enhanced Services on a Trial Basis" for instructions on how to do it using our conversion kit (the trick is...it comes with Junos on a USB already properly formatted...)
-Keith