There are two ways to add more accounts to an existing campaign.
If you created a campaign with a Salesforce list:
Simply edit your SFDC list to update your campaign. The target account list will update nightly, so your changes will reflect the next day.
If you created a campaign by uploading a CSV:
Click the "edit" button under the "Actions" column of the Campaigns index page:
Then upload another CSV file with your new accounts. The accounts in that list will be added to your campaign (e.g., will not replace your current list).
Make sure you click on "Next" until the last screen, then "Launch", and lastly "Save" to implement your changes. You'll notice that all of your previous settings are pre-selected for you.
If you fail to click "Save" at the end, your new CSV account list will not be added to your campaign.
If you have more questions, please contact us at firstname.lastname@example.org.