You should be able to link your RunKeeper App to the Withings app, but will first need to link RunKeeper to Apple Health. 

To link RunKeeper to Apple Health, please visit this helpful page. 

Once you have linked RunKeeper and Apple Health, you will need to link the Withings app to Apple Health. To do so, please follow these steps:

1. Open the Withings app
2. Go to your Profile.
3. Scroll down to the 'Apps' section.
4. Tap the Health icon. 
5. Tap Activate and then Next.
   The next screen will allow you to select the data you want to share with Apple Health.
6. Tap Turn On All to allow Withings to import and export all health data to Apple Health, or choose specific activity-related data to share including Steps and Distance. 
7. RunKeeper and the Withings app should now be linked!