The best way to learn to create Sigil plugins is to examine the code of existing plugins. There is also a plugin developer manual epub the can be downloaded from here:
https://github.com/Sigil-Ebook/Sigil...ork_rev14.epub
But I am happy to help get you started as I think this could be a useful plugin/addition for Sigil.
All we need is an example epub with a single chapter and its associated correct smil file. That way we can see exactly what you are trying to achieve.
It has just been too long since I played around with the smil spec to remember how they are integrated into the opf and what the contents should look like.
If you can attach one here, I would be happy to create a simple plugin example for you to modify and fix as you like. You can then add a gui to it to collect any extra information you need on the fly.