Doesn't it make more sense to have a WhatsApp backup file to Signal backup file converter rather than forking the whole app.
This fork would end up being a significant maintenance overhead for something that could be done more simply.
If the backup restore is a merge rather than a nuke and pave it might make sense to do whatever Molly[1][2] does to allow it to be installed alongside Signal, so it's install this, import from WhatsApp, then backup/restore without having to do the uninstall/reinstall dance.