I'm sorry, I misunderstood your initial question.
I thought you want ed to use this text area to send the purchaser a message using Universal Email, not have it sent in the email that paypal generates
Do not create a seperate form, in the form that sends the information to paypal, add a hidden form element:
<input type="hidden" name="on0" value="message" />
Then create a text area:
<textarea name="os0" cols="30" rows="8"></textarea>