Since the turn of the century, I have been writing about adding liquids to pre-made boilies a couple of days before use. This helps to open the pores of the bait to allow the important ingredients inside a boilie to start working their magic quicker.
The other advantage to adding liquid of course is that it makes the baits a little heavier for distance baiting.
There are few weeks that have passed by over the last 20 years when I have not been asked about what can be added. So here is a selection to cover (no pun intended) most types of boilie. You cannot overdo it with these liquids, they are a strength the carp will tolerate whilst also adding a quick flavour boost around your baited area