If you compose an email from your Mailboxes menu, your iPad will use your default email address in the "From:" field. You can temporarily change this without having to change your default address. This is also helpful if you start composing an email from one of your accounts and realize you wanted to send it from a different address.
All you have to do is tap on the "From:" field. A list of your registered email addresses will come up and you can select the appropriate one. Tap on the one you want then continue with the rest of your email.