You can add another of address or alias to send mail from your inbox. Remember you need to have access to that mailbox. You can choose which will be the default reply-to address.
1. Open Gmail
2. Go to Settings
3. Open Accounts tab.
4. From Send mail as click Add another email address you own.
5. You will be asked to add a new send as address. You will get verification email to that address
6. After you have authenticated the address you can send mail from that address within your Gmail.
When you are sending an email you can choose the send as address for every email you send: