Let me make sure this is what you want. You have a fasta file and the GFF annotation and you want to build a GenBank format file combining the two? Sorry to nitpick but since GenBank is a more complex format you are not extracting information here but synthesizing it from simpler pieces.
It also helps to search biostars before posting new questions: Converting Gff/Gtf + Reference To Embl Or Genbank ...Any Tools Available?
Edit: I am not sure why @Wouter asked you to post this as a new question unless the inference I am drawing above is incorrect.