Merging the Email Database and the Standard Database

From ancient times I have on the one hand in DTPO and now in DT3 an email database and on the other hand an additional database whre all the other documents are indexed in. Now, I want to transfer all the emails within the email database into this other database but having all email accounts should being kept associated to their real IMAP account as they are within the email database, but as well should have been transferred too all the emails from the database from ancient email accounts, which have currently no active accounts.

How can I get this?

There’s actually no real association to any account or client stored in the databases, archiving mailboxes just creates the same mailbox hierarchy in DEVONthink. In which location are the emails stored?