The only idea I could suggest is to decomposit this problem, break it into two (or more). Like:
- Figure how to export Whatsapp messages
- How to import messages to Signal
- How to bridge these two formats
There is also this GitHub - 42wim/matterbridge: bridge between mattermost, IRC, gitter, xmpp, slack, discord, telegram, rocketchat, twitch, ssh-chat, zulip, whatsapp, keybase, matrix, microsoft teams, nextcloud, mumble, vk and more with REST API (mattermost not required!), but it’s not exactly what you asked. Just in case.