It is possible to modify the server files
In the same way that it's possible to create any of these projects in the first place, or create your own startup sure.
In any way that's practical for someone who doesn't already realise how and have the skill set? No not really.
Not to mention that the licensing would be a nightmare, and that Spigot doesn't use the same mappings as MCP, so it would take someone a lot of time and effort to essentially port Bukkit back to Sponge like blood used to for Bukkit to Forge.
I'm locking this to prevent this turning into a flame war, and people thinking they can just patch stuff and be up and running.