Hi, I consider an LLM in International law at Fletcher school of law and diplomacy and maybe do one course of the program at Harvard. Unfortunately, I just found out that the Fletcher LLM does not qualify to sit the bar. Does anybody know what alternative I would have? Would it be possible to extend the programm at Harvard in order to be able to sit the bar?

no chance ...
only thing you can do is go to harvard and cross register up to 4 or 6 (not sure anymore) credits at fletcher.

If your goal is to sit the bar, you should do your LLM at an ABA-accredited law school. Fletcher is not one (the ABA does not consider it a law school).