That is a good question - there is an easy and a more complicated answer.
Easy answer: just model the packaging recyclate as input to the process that is producing the packaging, this works.
Complicated answer: The easy answer works only if you have set the amounts correctly, otherwise, you put two conditions on one process (production of a certain amount of product, with given input ratios, and with input depending on the production of the process), which cannot be satisfied at the same time.
Overall, thus, an easier solution might be to not close the loop for the recycled packaging, you will then have recycled packaging as input and recycled packaging as output, and can yourself calculate the net recycled packaging for your system. We are currently preparing an example case study on packaging where this will be addressed.