Yes, it is possible to change the default email sender within Campaigns.
To do this, go into your Email Account Settings and click "edit" next to the account you would like to establish as the default sender as shown here:
From there, check the box shown below to make this account the default sender:
Once updated, the default sending preference will be saved and applied to the account.
Once this mailbox is set as the default sender this address will be pre-selected when you start a new campaign.
💡 Please note this can not be done with email accounts connected by other team members and can only be performed with accounts owned by the user.