I don't use Windows and had a bitch of a time getting TerraOnion's convert tool working in Wine. So I wrote my own conversion tool, maybe other non-Windows devs will find it useful.

https://github.com/city41/neosdconv

It should work on any OS as it just requires nodejs.

It can convert some commercial roms but not all of them. The real intended usage is to convert homebrew roms for dev purposes.

TerraOnion allows you to do "patch" roms, ie just update only the P ROM for example. Haven't implemented that yet but it shouldn't be too hard.