You can't write headers to messages you receive.The only way is to download the message (including the three 78MB PDF attachments you got from that graphic designer), add the header field, upload everything and delete the first message.

But if your longish snoozes end at the same time, you can use a flag.

(IMAP's cache semantics say: If you download a part of the message it stays valid. Other people cannot change what you have downloaded.)