I have tried these options. Using Mac by the way.
I started setting up two accounts. Messages was downloaded, but a set of e-mail lists was created without being asked, and e-mail seems to have landed there The show also under the uppermost boxes, '' unread" etc.
I have about 30 subfolders, and if there are no way to import account settings and folder structure, I think I will have to wit untilm I have more time experimenting. But thanks for your help.