once you have the IMAP account added to the outlook setup,
you should be able to drag/drop folders/messages from one account into the
other.
(might be the right time to clean a few out)

you dont have to do them all at once do you.
once the messages are stored on the imap server:

if you dont use offline mode in outlook, only the message headers are
transferred over the network when checking mail ...

my 2 cents
wolfgang

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