Group policy prevents adding of URL's to IE trusted sites

Hi,

I have created a group policy that add's in URL's to the trusted sites section of Internet Explorer. After doing this users are prevented from adding any further URL's into this section of IE. The add button greys out.

I have tried to apply this group policy both for users and computers sides of GP with the same problem.

Does anyone know how I can use group policy to add in a list of mandatory URL's but still let the user have the ability to add additional ones?

Thanks.