No not according to the fluff of the HH series, which has to be honest, IMO turned into a disaster and I stopped buying them long ago.
He should have stayed possessed, it was much, much more tragic and was very Picture of Dorian Grey
esque in the original Fulgrim.
Ironically in a thread I started long ago (because the EC and Slannesh are my favourite aspects of chaos) I highlighted that based on reading A-D-B's Night Lords series you could actually draw an idea that it was hypothetically possible of him freeing himself:
Ironically when that change occurred it felt like it soured one of the best bits of fluff to come out of the HH series.
I mean, what is better than a Greater Daemon of Slaanesh inhabitting a Primarch's body, doing all kinds of crazy shit, retiring to a pleasure planet after the Horus Heresy and being elevated by Slannesh and mutating Fulgrims body into this:
Oh and all the while Fulgrim is trapped screaming, looking through his own eyes, unable to control his body all the fcked up shit this Daemon is getting up to in his name.
Sooo much better that way, and it makes him unique amongst Primarchs.