WPS: Editing MSTs or MSIs from Microsoft.

Don't bother. Microsoft has special tools to customize the installation of their Office products. The resulting MST files are in general not to be messed with. They have so many customizations that if you try to edit an MST file created by Microsoft's Office Resource Kit (ORK) with WPS, the resulting MST file is not useable.

I have seen some MS Knowledge base article explaining how to use ORCA to edit a MS Office MST file to embed the Serial number but that's it.