Based on your posting, I'm assuming that you are using Developer to add the registry keys to a component.
In the registry view, you right click on the registry entry to choose Install only and it is greyed out.
Did you need to modify the value for a given key? Windows Installer will create the key, if the key does not already exist and then add the value. If the value already exists, then Windows Installer will update the value. This is the default behavior of Windows Installer.
Could you please explain with an example, so I can better understand the problem.
thanks
alaks