Be sure you live your life, because you are a long time dead.-Scottish Proverb
In this case based on what you claim is on this circuit , technically you could add the fan's load to the circuit.
However, it's very likely the range hood requires a grounded circuit to meet it's safety rating and rightfully so.
I would recommend a new grounded circuit be run .
Btw --- just to third --- a GFCI doesn't need a grounding wire for it to operate.
"" an ounce of perception -- a pound of obscure "