So I’ve managed to do it.
Essentially, the issue comes from not knowing how to create the bundle with a name so it can be merged with the original array using merge(). So I’ve created a new array using the aggregator module and then created another variable which uses the merge() function of the output with the source array I wanted to add these new variables into.
Also realised the bundle which is added to the array doesn’t need to directly match the source array which I wasn’t aware of before. I expected it to need to have the same key-value pairs to work.