Sorry not possible to do.
You will need to compose the messages on the BB, once you turn your antenna back on when you are grounded the messages will then send out.
If you have a lot of emails to catch up look into getting a portable Bluetooth keyboard. Here is a link to one of the sponsors on the forums that sells a very good product: Freedom Input Ltd - Home
Make sure your BES admin however allows you to connect this type of external device to your BB as they have the ability to turn this functionality off on your device.