You can copy and paste the messages from an email to an email form, or to the message board at Groups.io. Both options mean resending them.? I have one group I am moving manually that has about 100 messages are less that I want to move. I have been copying and pasting frpm Yahoo board to my new group's message board.? Only the ones worth seeing again.
You could also try forwarding them, but that also will be reposting a message your group has already seen. Depends on how valuable they are to the group and their value for future reference.
Janis
Janis