When you subscribe to an IMAP account's drafts folder, it doesn't work "as expected" in Outlook. The messages contained in the folder will not be editable in Outlook because Outlook sees the messages as sent messages, not drafts. Moving the draft messages to Outlook's local drafts folder won't make a difference; Outlook still sees it as a message that was already sent. You can go into Edit message mode but the Send button will be missing.
The solution? Open the message and use the Actions > Resend Message command.
If you save a message opened using Resend message command as a draft, it will go into Outlook's local Drafts folder, not the IMAP drafts, as this is the folder Outlook uses for its drafts. Once the message is sent, you'll need to delete the draft message from the IMAP account's Drafts folder.