The short answer is: No, there is no way to include pixel tracking after a campaign has been published.
It is very easy to just copy the previous campaign and add your pixel tracking from the creation form at the bottom of step 2: ads design!
It is not possible to change a campaign's objective after it has been published (on Facebook or on AdEspresso) which is the reason for the lack of flexibility here.
It is very important to have the right objective set for the campaign, so we will always recommend you to simply copy your campaign and make a new version of it if you have fresh pixels you need to be tracking.
You want to know more about Objectives? Have a look here!
Looking for more info on pixel tracking?
Our blog is a great resource: