custom Imap flag

I am trying Aspose Email ImapClient to fetch messages and do some processing. After that, I'd like to mark a flag on the message so that they are skipped at next fetch. It seems custom flag is not supported in ImapMessageFlags?

Java Mail has that. https://javamail.java.net/nonav/docs/api/javax/mail/Flags.html#Flags-java.lang.String-