I deal with Bitcoin Script and its opportunities.
Can stack-based language Bitcoin Script be transformed in a turing complete language by doing a hard fork in principle?
No. (Without hardfork)
But everything can be implemented by next segwit script versions (today we use segwit_v0) if community votes for it
The right question is: why do you need turing complete language and scripts in bitcoin?
Does segwit now allow to add a different script language? So i think the answer should be yes.
The current segwit (v0) does not allow anything except old bitcoin opcodes