When an email campaign is created, The email address that the campaign comes from must have already been added as a contact and have the email address approved before being able to add that email address in. If the email address is not approved, then you will not be able to select the email address chosen. If you want to know how to approve email addresses, then click here.
Once this has been added, then the 'from' email address will show with a drop down menu during the process of creating the campaign. This will enable you to choose which email address the campaign is coming from. This will allow you to change the 'from' email address when sending the campaign. The reply to email address can be over written easily by simply clearing the field and typing in the desired email address.