It should be possible with imapsync, see https://imapsync.lamiral.info/FAQ.d/FAQ.Folders_Selection.txt
In that case you’d need to use a custom script, it’s not possible in the UI, see GitHub - NethServer/ns8-imapsync: Imapsync for NethServer 8
There’s also a fetchmail community app but it needs to be configured on CLI.
See Howto manage or customize NS8 podman containers for a general documentation about podman containers.
To enter for example the dovecot container to get the directory structure:
runagent -m mail1 podman exec -ti dovecot sh
The mails are stored in /var/lib/vmail
Yes, that seems to work but you have to care about permissions, see Sieve 'fileinto' Public Mailbox gives "Permission denied" - #7 by hinzberlin