Arrrg! Simple but it took me a while to find it!
On my SMP7 (NS7), there is a folder "D:\Program Files\Altiris\Symantec Installation Manager\Installs\Altiris\"
I edited the "migration_setup.vbs" script and I found the list of all the msi required for the Migration Wizard installation.
I had already copied all files from "D:\Program Files\Altiris\Upgrade\" from my SMP7 (NS7) to my NS6 same path.
I copied "D:\Program Files\Altiris\Symantec Installation Manager\Installs\Altiris\altiris_nsupgrade_7_0_kb49812_sp5_x86.msi" from my NS7 to same path to my NS6.
After running "altiris_nsupgrade_7_0_kb49812_sp5_x86.msi" on my NS6, the Migration Wizard was showing, among others, "Exports Software Delivery Packages along with its Programs" exporter that I was missing since beginning.
All documentation I found was talking about a "%ProgramFiles%\Altiris\Symantec Installation Manager\MigrationPackage" that does not seem to be existing anymore and that was pointing me on a wrong direction.
I hope that this will help someone else.
RJ