I'd like to add a url parameter such as exp=4tabA (when user enters experience A) and exp=4tabB (when user enters experience B), etc. The goal is so I can differentiate between revenue made within experiences in my Drupal CMS reporting (not just Target reporting).
I'm guessing the page needs to load with the new parameter in order for your CMS to pick it up. If this is true you might need to use a redirect activity on top of your normal activity.
Activity 1 (redirector) send 50% of people to same URL with this query param exp=4tabA and the remaining 50% of people to same URL with this query param exp=4tabB appended.
Activity 2 contains the actual content changes for the 4tabB experience (assuming 4tabA is default and needs not content changes). You will need to use the entry page URL with the exp=4tabB included so people qualify for the activity appropriately.
This isn't ideal though. Redirects shouldn't be used in this manner in my opinion because they can add page load time to the end visitors. It would be preferable to find another way to send info to your CMS. As Amelia has mentioned you can use response tokens to return info about the activity/experience a visitor saw to the page. You just need a mechanism for capturing that and sending it to your CMS - maybe an image request or something similar could work.