Looking at Josie's dialogue in that scene:
Imagine dying frightened and in pain, and having that as the only part of you which survives.
I wouldn't like that at all. Ventress wants to face it.
You want to fight it.
But I don't think I want either of those things.
her becoming part of Area X seems more like a passive lack of resistance, letting nature take its course, rather than an active will to join.
In the book the film is based on, there are frequent references to the biologist actively fighting the growing sensation inside her (the 'brightness'), strongly hinted to be her 'becoming' part of Area X, so this interpretation makes sense from the book's perspective:
The brightness infecting my senses had spread to my chest; I can describe it no other way. Internally, there was a brightness in me, a kind of prickling energy and anticipation that pushed hard against my lack of sleep. Was this part of the change? But even so, it didn’t matter—I had no way to combat what might be happening to me.
To keep the brightness in check, I would have to continue to become wounded, to be injured. To shock my system.
When I finally picked up my husband’s journal and started to read, the brightness washed over me in unending waves and connected me to the earth, the water, the trees, the air, as I opened up and kept on opening.