How to add new string to Registry Entry of type REG_MULTI_SZ?

I have 1 registry entry of type REG_MULTI_SZ. This entry already contains some string in it.

Now i want to add 1 more string to it by using .net classRegistryKey . this class has method key.SetValue(string,string). But when i use this method it removes all strings which are already there and then inserts new string, in short it overwrites. I don't want to touch strings which are already there, just want to add new string at the end. Anybody know how can we do this in C#.