[Dovecot] Support of client specific flags?

Gnus use a lot of custom flags to represent the messages state wrt. Gnus.

How well are these flags supported in dovecot? How fast are access to them? (Ie. will dovecot have to open the message and parse the headers)

Are they supported when using imapsync to transfer messages from a different IMAP server? (I guess I'll find out soon, because I'm using imapsync to transfer messages from a cyrus imap daemon to a dovecot one, as I'm writing this)