Thanks for the suggestions, but I need to do this after users have been created.
Would this be the best way?
To run this for each user:
doveadm mailbox create -u $username -s Drafts
doveadm mailbox subscribe -u $username Drafts
doveadm mailbox create -u $username -s Junk
doveadm mailbox subscribe -u $username Junk
doveadm mailbox create -u $username -s Sent
doveadm mailbox subscribe -u $username Sent
doveadm mailbox create -u $username -s Trash
doveadm mailbox subscribe -u $username Trash