Or making small additions in an enclosed way. Had a similar idea on my drive into work today.
It's funny that you mention that. We already have this product being designed at the moment. So it will probably come into the market soon. We also have some 2.5L PET bottles too and you probably would have seen these inexpensive carbonation caps on the website too:
https://www.kegland.com.au/plastic-carbonation-line-cleaning-cap-fermzilla-cap.html
So once we have the combination of these parts we will have a very inexpensive way for someone to get into kegging.