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No. It is also considered rude to send a join request without asking permission first.
What I reccomend is that you ask if any bombers want a gunner, or if anybody wants to gun your bomber. But don't ask too frequently. Spamming the text buffer will actually probably make it less likely that somebody will take you up on your offer.
If you don't get a response, fly a mission alone. Then ask again ince the mission is over.