During pranayama is there something to be said about how one can enter Nirvilkalpa?
Yes. By monitoring the breathing and watching the object of meditation become focused the breath or consciousness and “energy” will as you breath in towards maximum move up through the spine moving into the throat and into the cranium. It is this passage moving fully up the central channel or shushumna that allow Samadhi to take place.
By sitting in Samadhi and focusing on nothing or the object of meditation (one or the other) the mind will relax (mental distractions will slow and stop) and the higher or deeper need for full body relaxation will take you higher into cosmic consciousness (nirvikalpa Samadhi).