When sending email from the native messages app on the PlayBook, you can choose which account to send it from by scrolling up to the "From" drop down menu and selecting the email account you want to send it from. Of course, this is providing that you have added the desired account to your PlayBook.
I think the OP is referring to sending email with a different 'Sent From' alias.
I too want to send email from my Gmail account, but want the recipient to see that its been sent from my personal domain that is used for email forwarding.
My domain address is simply an address - there is no email hosting associated with it.
I send from my ISP email address using Outlook aliased to my own domain from my home PC. when using Gmail (from my phone or a normal browser on a laptop) has the option to "send mail as"