Hi,
I believe grafting is possible. But i recommend you use cultivars from north africa and middle east (like: "Nabali", "Piccolin Marroquin", "Zeituna") that will probably adapt better to the Ethiopian climate.
As for a grafting method, since we are talking about grafting adult trees I suggest you use patch budding. You should read this file:
http://fruitsandnuts.ucdavis.edu/cro...ding_08_05.pdf
Also I suggest you contact the Olive Institute in Tunisia (
Olive Tree Institute (IO) ), maybe they will be able to give you plants and some more info on the cultivars.
Cheers,