The "Adding an External Number" option will allow you to add outside numbers if applicable to you, but you cannot customize how the numbers come up for your contacts. They will still appear as a regular phone number on your contacts' screen. This is only applicable to landline numbers you already have that you would like to use. As long as the numbers you add have the ability to make outbound calls AND receive inbound calls you can add them here instead of purchasing phone numbers through BatchDialer.
Once your phone number(s) have been successfully added, they will show up in your list of phone numbers. Keep in mind that inbound calls won't be processed by BatchDialer and when someone calls you back they will be sent straight to that phone number. For example, if you added your cell phone number to make calls, if someone calls you back, you will simply receive the call on your cell phone.
Finally, if you have the "advanced phone reputation" add on, you must remember to manually add the phone number to monitoring if you wish to do so! Simply select the phone number, click "actions" and select "monitor reputation".