If you'd like to use pixels that are not available to add at RocketLink you can do it via Google Tag manager.
Other use cases of this setup are using multiple pixels of the same type in the same link or customizing your pixel's event.
Here is an example on how to use Google Tag Manager with your pixels -> LINK
You can do the same with almost any other type of retargeting/analytic script.
After creating a Tag in GTM you will need to paste your GTM ID to RocketLInk.
No matter how many scripts you add to your GTM, one GTM ID utilises only one pixel slot at RocketLink.
So this is also a great way to save your slots!