Birkath haShanim

May I use the nusach of birkath hashanim as it appears in nusach eretz yisrael if I am davening nusach Ashkenaz?

One may indeed do so. Given the fact that the birkath hashanim of nusach eretz yisrael contains language more relevant to the climatic conditions in Eretz Yisrael doing as you suggest is even advisable.