facebook/react-native#1908 (comment) @alinz, @brentvatne & @jaygarcia
So lets say you want to add eval method to WebView component. What you have to do is: use category feature in objective-c.
// RCTWebView+WebViewExBridge.h
#import "RCTWebView.h"
@interface RCTWebView (WebViewExBridge)