Trying to mix and match brands like that is always a risky proposition: specs don't have to be off by much for things to not fit properly.
That being said, WE M9 mags work in the KJW so it seems a fairly safe bet the specs are pretty close on both (i.e., Marui clone).
But if you're just going for the look of that railed frame, why not stick to WE?
http://www.airsoftgi.com/product_inf...oducts_id=6089