It is not supported. You can only add an IMAP mailbox on the mobile app and not on gmail.com. The IMAP account is then displayed as an inbox completely separate from your gmail inbox. There is no pull and no integration.
This isn't the same thing. Yes Gmail provides IMAP so you can read it from other clients. The issue here is that Gmail cannot use IMAP to ingest email from other accounts, as it can (or could) using POP3.
