Still exploring this but AFAIK you can put .sfs files renamed to .sb in the modules folder or just in the same folder as the alpha_12 and extra_12 and they will be loaded on next boot.
The nice thing about this system is that you can make a .sb "override" another by renaming it so that it becomes further in the alphabet.
You want to change /root/.start and/or any other files that are in alpha_12.sb or extra_12.sb, then create e.g. zz-mychanges.sb and it will overwrite all existing files.(which is not the case with .sfs's loaded with sfs_load, it's just an addition and won't overwrite existing files)
Hope this is clear explanation (my english is not very well)