You can add alternate sending addresses to Gmail from the settings. Keep in mind that you'll have to own the alternate address to use it. You can also set the default sending address.
1. Open Gmail's settings
2. Go to Accounts and Import tab
3. From the "Send mail as" click Add another email address
4. Add the address you want and own. A confirmation email for you to use the address is sent to the address. If the added address is primary address' alias, the email should arrive to your inbox. Click the link on the email to confirm the ownership of the address
5. You can now send emails using the alternate address
Sending address can be changed per email by clicking the From. You can also choose the default address to be used for sending from Gmail's settings by clicking "make default" next to the address.